The seedy underbelly of any baseball roster…
If you think the opening day bullpen is the one that will be used for 162 games, you’re sorely mistaken. Injuries happen. Relievers are streaky. One team may discard a fringe MLB pitcher, and another likes their stuff enough to add them to the roster. Trades and prospect promotions are inevitable as well. Not every reliever is a closer. The nature of the position allows you to get by with one or two above-average skills. The Red Sox used 25 relievers last season. They’ll probably use another 20-something this season, including many names that aren’t on this list right now. For now, here’s a look at some of the names that will be in the mix for bullpen roles to start the year.
Zack Kelly
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Zack Kelly is one of the most interesting arms in the Red Sox bullpen. He has fantastic stuff, but the performance doesn’t match. In 56.2 innings last year, Kelly’s ERA was just shy of four. Kelly uses four pitches, and each of them has positive traits. His four-seam has all of the elements of a good fastball. He throws hard, averaging 96.5 MPH but reaching 99 MPH. He uses his 6’3” frame to get seven feet of extension, increasing the perceived velocity. The shape is also great. All of this should combine to make an excellent pitch, but it doesn’t. The strike rate is only about 60%, while the swinging strike rate is just 10%. The pitch does have a high ground ball rate and has been described as “heavy” by SoxProspects.com, but you’d expect more swing-and-miss from a fastball like this.
His sweeper is his next most-used pitch and might be his best. Right-handed hitters are baffled by it. He throws it both in and out of the zone, gets called strikes and whiffs, and doesn’t allow hard contact against it. Stuff+ gives it a 132. I give it two thumbs up.
Kelly throws a cutter as well, and while it’s not a devastating offering, it’s the perfect “bridge pitch”. The movement profile sits right between his fastball and sweeper, as does the velocity. He does a nice job of throwing to the glove side consistently. Righties take a ton of called strikes, likely due to the threat of a sweeper that breaks an extra fifteen inches. Lefties don’t do well either, taking called strikes, whiffing, and hitting it on the ground frequently.
His last pitch is his changeup, and it’s a confusing one to analyze. He throws it 35% against left-handed hitters and 10% to righties. With that high usage against left-handed hitters, you’d expect a higher strike rate. Even with the low strike rate, though, lefties only hit 0.182 against it. He locates it well but only gets an average amount of whiffs. I almost wonder if it’s too different from his fastball and hitters pick it up well or if his fastball isn’t enough of a threat to induce more swings on the changeup.
All of this sounds pretty good, but Kelly’s ERA was still middling, so what gives? Simply put, Kelly walked too many hitters: 9% of righties and 14% of lefties. The fix against righties is simple. Kelly needs to get more strikes with his fastball. He’s working on adding a sinker, which could help get additional called strikes, but it ultimately depends on command. Against lefties, I’m looking at the changeup. He’s throwing it 35% of the time but only getting it in the zone on 33% of pitches. Kelly throws his changeup in the zone to righties on almost half of his pitches (to his detriment). Improved fastball command probably makes everything else work better, but being able to zone his changeup more to lefites would also likely decrease walks. Kelly still has options and could start the season in the minors with a crowded bullpen, but his stuff is too good to live in Worcester.
Luis Guerrero
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Luis Guerrero’s BaseballSavant page lights up like a Christmas tree. Just about every category shows 90th percentile or better (besides extension, that’s comically low). He only pitched 10 major league innings last year, so I want to pump the breaks and look at his pitch mix to see how legit this is.
It’s a fastball, changeup, and slider mix for Guerrero, although he has thrown a cutter and sinker in the minors. His fastball is hard, getting up to 99 MPH. It isn’t an outlier in terms of raw movement, but it has unexpected movement, given his arm angle. Here’s a visual of Guerrero’s fastball via Max Bay’s dynamic dead zone app.
The pitch gets about 16 inches of vertical movement and ten of horizontal. Neither is out of the ordinary, but they’re unexpected when you consider the release angle. The pitch rises and runs more than expected, leading to whiffs. In his very limited stint in the majors, right-handed hitters had a horrible time against it. I wouldn’t say the command was good, but Guerrero landed the pitch in the strike zone, and the characteristics compensated for less-than-ideal locations.
Against righties, he opted to use his slider as his secondary pitch. The pitch performed well in the minor leagues, but command was inconsistent in his short major league stint. If he can command it, there’s no reason it can’t be a good secondary offering. He used his changeup against lefties, and while the results weren’t great, the pitch passes the eye test. It spins similarly to his fastball and gets a ton of arm-side movement. He also showed a good feel for it, consistently landing it away from lefties.
Guerrero’s fastball is strong enough to get by as a reliever, even if his secondaries are just average. He’ll need to command his pitches, but his ability to punch out hitters will help him carve out a role in the bullpen.
Greg Weissert
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Greg Weissert’s season-long line doesn’t tell the story of his season. The way I see it, we can break Weissert’s season down into three (or four) parts.
March 30 – June 25: 34 appearances, 2.65 ERA
Weissert was used very frequently early in the season, and he excelled. He was a very difficult at-bat for righties, using his sinker for early called strikes, and then four-seamers and sweepers to put them away. Lefties had a better time against Weissert, but he was still able to get in good counts and use his sweeper effectively to punch them out at a high rate. He did have some early season struggles with inherited runners that don’t show up on his ERA, but he was otherwise effective.
June 28 – July 28: 11 appearances, 8.53 ERA
Okay, I did pick a fairly arbitrary cut-off date, but this is right around when the wheels started to fall off for Weissert. Over these 11 games, Weissert walked about 14% of the hitters he faced. It’s not as if hitters figured Weissert out suddenly; his command just suffered. He was in the zone more often with his sinker, but the ball was middle and glove-side more frequently. He adjusted by throwing more sweepers, but it wasn’t as effective with more usage. Lefties, in particular, stopped whiffing on sweepers. His strikeout rate fell from 27% in his first 34 appearances to 15% over this span. More balls in play is rarely ever a good thing.
This wasn’t exactly unexpected. It’s not that Weissert overexceeded; he just has traits of an inconsistent pitcher. His delivery is very cross-body, which can lead to unstable command. He was also leaned on a lot in the early part of the season, and he may have been run down. He was optioned to Worcester to reset and spent the next month there.
August 20 – September 28: 17 appearances, 0.00 ERA
When Weissert returned, he was excellent. His strikeout rate didn’t rise to the insane early levels it had, but he only walked three of the 62 hitters he faced. He also didn’t allow a single extra base hit. He kept the ball in good locations and let his stuff do the rest.
Weissert isn’t going to be a closer, but he serves a purpose in the bullpen. His arsenal makes him an incredibly tough at-bat for right-handed hitters, and he can pitch for either a strikeout or a groundball. On the other hand, he’ll probably always have difficulties for lefties. His sweeper won’t play, and sinkers typically don’t excel against opposite-handed hitters. He’ll also probably be streaky due to his mechanics, but he’ll be a useful reliever for parts of the season.
Josh Winckowski
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Before I write one of these blurbs, I go through five or six different websites, poring over stats, interviews, and whatever else I can find to make sure I’m telling the right story about a pitcher. I write my thoughts in a notebook and then reference the notebook as I write to make sure I don’t miss anything. I’m saying this to apologize in advance for having more to say about Josh Winckowski than anyone needs to read. Let’s do it.
Winckowski is often the target of internet outrage. I’d like to say it’s unfair, but relievers who don’t get the job done are always going to be low-hanging fruit on which to place blame. Winckowski had a solid 2023 season, appearing in 60 games and posting an ERA below three, but regressed in 2024. His strikeout rate fell from 22% to 17%, and the lefties that he was so tough on in 2023 (.609 OPS) figured him out in 2024 (.807 OPS).
Winckowski has some traits of an effective reliever. He throws three fastballs: a sinker, cutter, and a four-seam, each with good velocity. He also throws a slider and a changeup as an off-speed pitch to use against each side of the plate. Unfortunately, none of the pitches are good enough on their own for Winckowski to get by. His sinker performs well when he throws it inside to right-handed hitters, but he doesn’t spot it there often enough. Whether that’s a gameplan decision or an inability to throw to the arm-side, I don’t know, but something needs to change. His cutter was productive against lefties in 2023 as he threw it inside often, but in 2024, too many pitches stayed over the middle of the plate, limiting their effectiveness.
His fastball command is one issue, but command is finicky in small samples, so he may improve in that regard. The other major issue for Winckowski is how his entire arsenal fits together. In theory, it should work. He throws all three fastballs, a slider, and a changeup to cover the entire plate. However, when we look at the mix as a whole, there are some glaring problems.
As you can see on the left side of the graphic, his slider and cutter blend together. With similar movement profiles, it’s less of a threat to hitters and limits the swing-and-miss potential. On the other side, we see something similar with his changeup and sinker. Some pitchers succeed with this combination. Brayan Bello is an example. The difference is that Bello’s changeup is about eight miles per hour slower than his sinker. Winckowski only has a 3.5 MPH difference. Even when hitters are expecting velocity, they can adjust their swings to fight off his changeup. Winckowski used the slow ball 20% of the time against left-handed hitters. Over 20% of those changeups were fouled off. The pitch did have an 80% groundball rate and wasn’t hit hard, but lefties hit .400 against it. Some batted ball luck hurt the productivity, but the current shape caps how many bats Winckowski will miss.
All is not lost for Winckowski, though. His spring training performance thus far hasn’t been great, but who cares about spring training stats? In his most recent outing, nine of the ten sinkers he threw landed on the inner half. That’s not to say he has great command all of a sudden, but at least throwing arm side sinkers appears to be part of the game plan. His breaking balls also went through some tweaks. His slider and cutter still blend together, but it looks like he may be working on a sweeper as well. He didn’t face any lefties on Saturday and didn’t throw any changeups, so the jury is still out there. Either way, it seems as if Winckowski is attempting to adapt. He still has an option year remaining, so he can be sent to Worcester if necessary. He’s shown to be a durable arm who can provide length, so even if he isn’t always lights out, he does have some value.
Adam Ottavino
Photo by Maddie Malhotra/Boston Red Sox/Getty Images
I’ll keep this one short and to the point. The Red Sox brought back the 39-year-old as a non-roster invitee. At this point in his career, he is who he is. He throws from a low three-quarter release, giving right-handed hitters an uncomfortable look. He throws backdoor sinkers for called strikes, huge sweepers over the outside edge, and a cutter to bridge the gap between the two. It works for him against righties but not against lefties. I’m not going to sit here and play pitch doctor with Ottavino. He likely is who is he at this point, and that’s okay. If they want to keep him on the roster as a veteran presence in the bullpen and to face pockets of right-handed hitters, that’s cool, I guess.
He can’t hold runners to save his life, though. That’s an issue when he gets into trouble. I’m not telling you any new information here. Moving on.
Austin Adams
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Austin Adams is another non-roster invitee, and he’s a weird one. He throws his slider 75% of the time. He says he does that because he “enjoys having a job in baseball”. Go ahead and read the rest of that interview with Fangraphs. He explains how he pitches much better than I can.
If you don’t want to read that, he says he throws the slider because it’s his best pitch, and it’s actually multiple different pitches. What’s great about the slider is how hard he’s able to throw it. Despite having a ton of movement, he still averages 87 MPH. It can win both in and out of the zone. He’s also adding a sinker, hopefully to get more called strikes. If he can limit his walk rate, he could be a very effective bullpen piece. The slider really is nasty.
Isaiah Campbell
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Isaiah Campbell was on the 2024 opening day roster. If you have any memory of him at all, it’s probably not a fond one. He pitched in eight games and allowed 12 runs. Ignore the ERA north of 16 for a minute, and look at his 2023 line: a 2.83 ERA over 28.2 innings.
I still believe that Campbell can be an effective reliever. His fastball has shown the ability to induce swings and misses at the top of the strike zone. His slider also showed promise. It induced tons of ground balls and got whiffs against both sides of the plate. He ran into trouble when he left his fastball over the plate, but who doesn’t? He missed most of 2024 with shoulder and elbow injuries, but his fastball velocity is where it was previously so far in spring training. I’m not expecting Campbell to be a closer, but he could be an okay bridge to the later innings if he’s called on.
Michael Fulmer
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The next name on the list of bullpen options is Michael Fulmer, one of the Red Sox latest injury reclamation projections. Fulmer, a former Rookie of the Year, didn’t pitch in 2024 after undergoing elbow surgery. He’s signed to a minor league deal, so he can be kept in Worcester in the event that he isn’t on the opening day roster.
Fulmer is a long way removed from his Rookie of the Year campaign. He also won the award as a starting pitcher but made a move to the bullpen several years ago. His performance has ranged from excellent to mediocre, but at one point, he was solid enough to handle ninth-inning duties for Detroit.
The last time we saw Fulmer, he was pitching for the Cubs. He made 58 appearances with a 4.42 ERA. Fulmer showed his strikeout ability, punching out 27% of opposing hitters. Against righties, his fastball and sweeper racked up whiffs, while his cutter consistently earned called strikes. There was maybe some batted ball luck, but not an amount that screams regression. Lefties punched out at a high rate but found their way on base as well. He threw almost 50% four-seams, and they didn’t work. Lefties hit 0.395 against them. The Red Sox hate bad fastballs, so I’d expect Fulmer to turn to his cutter as his primary pitch against lefties. He has an incredibly deceptive changeup that gets tons of whiffs, but the last time we saw it, it rarely ever found the strike zone, which limits its utility to two-strike counts.
Fulmer is returning from Tommy John, so don’t expect too much right away. His velocity was down in his first spring training appearance but has ticked back up throughout camp. I wouldn’t be shocked to see Fulmer on the big league roster at some point this summer.
Nick Burdi
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Nick Burdi debuted in 2018, pitched for three different teams, and amassed a whopping 25 career innings. Over those 25 innings, he posted a 6.48 ERA and 15% walk rate. He also can’t stay on the field, having every surgery imaginable from Tommy John to an emergency appendectomy. Somehow, he keeps finding his way onto rosters, though, so what gives?
The explanation is fairly simple. He throws 98 MPH from a three-quarter delivery. He throws a slider close to 90 MPH as well. That’s back end of the bullpen stuff. History has shown Burdi can’t stay healthy, but what if he does? With just a hint of command, he could be a high-leverage arm. He’s on a minor league deal, so there’s no risk involved whatsoever. The deal didn’t include an invitation to major league camp, so he’ll be in Worcester to start the season. Come back here if and when he finds his way to Boston for his annual 6.2 innings.
Wyatt Mills
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Wyatt Mills isn’t going to make the opening day roster. I’m going to write about him anyway because I like Wyatt Mills. The gist of my love for the righty is that he’s a John Schreiber clone, and he’s how I learned about the importance of attack angle. Mills throws from an incredible low release point, creating an extremely flat attack angle. This makes it very difficult to square up his fastballs. His sinker gets a ton of horizontal movement, creating an uncomfortable look for right-handed hitters. He hasn’t seen game action since 2022 due to arm injuries, but he’s going through camp and has been impressive thus far. I’d love to get velocity readings, but they aren’t available at this point. Either way, Mills may see some time on the active roster at some point this year.
Robert Stock
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Stock, another reclamation project, has already been assigned to Worcester. We won’t see Stock until later in the season, if at all, but he’s a good Twitter follow, so he makes the list. Congratulations, Robert.
Best of the Rest
This is a long list of pitchers who may or may not pitch in the major leagues in 2025. It probably seems like overkill. At the same time, the Red Sox used 25 different pitchers in relief last year, as well as two position players. So here are a few more names to know.
Noah Davis has been up and down with the Rockies over the last two seasons. Up and down with the Rockies is rarely a good sign, but his sinker has picked up and extra 2.5 inches of horizontal movement in camp. That might be something.
Hobie Harris was a minor league Rule 5 draft pick and comes with major league experience (I’m not sure how that makes sense, don’t ask). He threw 50% splitters with the Nationals last season. It didn’t work, but it’s cool that he tried.
Christopher Troye is a prospect who gets huge vertical movement on his four-seamer and tweets like he’s a LinkedIn influencer.
my clarity is flawless, my self-belief delusional, my discipline unshakeable, & my focus sharpened – there is not quit, only grit.
— Christopher Troye (@christophtroye) September 6, 2024
Yovanny Cruz was in big league camp and has touched 99 MPH with his sinker.
Josh Nunes throws nothing but off-speed pitches because who needs a fastball? Neat!
Jacob Webb has a high vertical heater from a low release point and huge extension. He’ll wind up in Tampa Bay and throw 45 innings of 2.63 ERA baseball.
Brian van Belle and Grant Gambrell both exist and have thrown a couple of innings. One of those names is actually just my friend from college and not a professional pitcher. He’s getting married this summer. We’re so deep in the weeds I can make things up, and nobody knows. Thanks for reading.