A fan at the Verizon Center posted video of the exchange on Instagram. In the video, Thomas can be seen directing a profane remark at the fan in question.
"We are aware and reviewing," NBA spokesman Tim Frank told Boston.com on Tuesday.
According to the individual who took the video, the fan was heckling Thomas, who was on the bench at the time, throughout the game.
Thomas pointed at the fan and responded with a "I will f--- you up, and you know that."
The Celtics lost the game 116-89 and also lost Game 4 on Sunday to even the best-of-seven Eastern Conference semifinal series. Game 5 is Wednesday night in Boston.