One thing that may have affected IT is that he really is not a sixth man in the true sense of it. After he joined the Celtics he pretty much became the best player on the team. Just because the coach put him on the bench does not make him the a sixth man. You cannot put Westbrook or Durant on the bench and then grumble when some refuse to buy your 'tactic'.
I am not taking anything away from IT, in fact I am giving him a bigger honor when I argue that he means more to the Cs than just a 'role' player. He is the MAN on the Cs.