Marcus Smart let his emotions get the better of him in the Boston Celtics' meeting with the Washington Wizards on Tuesday.
An animated Smart was filmed waving his towel during a heated exchange with assistant coaches in the fourth quarter of the 123-108 defeat.
The 22-year-old did not have many things go right as he scored 13 points on four-of-13 shooting.
It was yet another poor effort from Smart, who has made just 25 per cent (13-of-52) of his shots from the floor in his last five games.
After the match, a much calmer Smart told reporters players and coaches get "real passionate about the game". He added that he wanted to go back into the game, but the coaches felt better about him being on the bench.
The guard then went one step further and apologised on Twitter.
Smart wrote: "To my coaching staff and my team-mates and the Celtics organisation - I apologise for my actions tonight.
"In hating to lose, my actions were childish and unprofessional and never under any circumstances do I have the right to act that way and for that I sincerely apologise.
"I'm a role model to kids ... I'm sorry."