The deal is surprising considering Brown, 22, is coming off a disappointing season. After making the All-Rookie team during the 2016-17 season, Brown showed growth in his second year, averaging 14.5 points and 4.9 rebounds per game. Those figures fell to 13 points and 4.2 rebounds per game last season.
On top of that, the Celtics haven’t been eager to sign players on rookie contracts to extensions lately. Brown is the first Celtics player since Rajon Rondo to receive a rookie extension.
With the move, the Celtics are betting on Brown bouncing back after a tough third season. Despite being in the league for three years, Brown is still young, and has plenty of room to grow as a player.
Getting back to his sophomore numbers would be a step in the right direction. With this deal, however, the Celtics are making it clear they expect more from Brown moving forward.
More from Yahoo Sports: