A few months after putting the NBA champion Boston Celtics up for sale, majority owner Wyc Grousbeck was in the Middle East pitching the team to one of the region’s biggest investors. The Celtics played preseason games in Abu Dhabi in October as part of the NBA’s third annual trip to the capital of the United Arab Emirates. During that trip, Grousbeck met with Mubadala Investment Co., one of the country’s sovereign wealth funds, to discuss a potential investment, according to people with knowledge of the matter.