The former Texas Longhorn posted 12.0 points and 10.1 rebounds in 2019-20.
The 29-year-old is a throwback big man who's comfortable at home in the paint. He's a consistent double-double threat capable of scoring and rebounding in bunches on his best nights.
He's also excelled on the defensive end, ranking 21st out of 72 qualified centers in defensive real plus-minus in 2019-20, according to ESPN.com.
Thompson was also a key cog on the Cavs' four Eastern Conference championship teams from 2015-2018 and averaged 10.3 points and 10.1 rebounds in Cleveland's 2016 NBA Finals win.
However, the rebuilding Cavs are going in a new direction based off a young core featuring the lottery pick backcourt of Collin Sexton and Darius Garland. The veteran Thompson doesn't fit that vision.
The 6'10", 238-pound big man has missed 68 games over the past two seasons because of injuries, but he was on the court for nearly the entire 2019-20 campaign. A healthy Thompson can still be a valuable contributor on any team given his production in the paint.
Now he joins a contending Celtics team that finished two wins away from making the NBA Finals. He figures to see plenty of playing time down low as Boston looks to make the championship round for the first time since 2010.