On Wednesday, theScore NBA tweeted the following photo from a video that featured fans setting the Thomas jersey ablaze:
In response, Brown tweeted, "This is pathetic."
Thomas spent parts of the past three seasons with the Celtics and developed into one of the NBA's most dynamic offensive players.
He was named an All-Star in each of the past two seasons, ranked third in the NBA with 28.9 points per game in 2016-17 and finished fifth in the MVP voting.
Despite that, the 28-year-old became an expendable piece in the Irving trade since his contract expires at the conclusion of the 2017-18 campaign.
Celtics general manager Danny Ainge also acknowledged the hip injury that caused Thomas to miss time during the playoffs played "some" role in his decision to make the deal, per ESPN.com's Jeff Goodman and Adrian Wojnarowski.
Brown is now just one of four players remaining from last season's Celtics team that finished first in the Eastern Conference during the regular season and reached the Eastern Conference Finals.
Brown was the No. 3 overall pick in the 2016 NBA draft out of the University of California, and he averaged 6.6 points and 2.8 rebounds per game as a rookie.
He figures to combine with rookie Jayson Tatum in 2017-18 to give Boston one of the most promising young wing tandems in the NBA.