He can't get to the rim until the defenders have to come out, contest his shot and he can drive the close out. Him randomly cutting and mucking up the spacing for everybody isn't going to help.
His offensive rating is bad because he has played a lot of garbage time on units that have been utterly inept on offense (for which he does bear some responsability). Per cleaning the glass, once you filter out garbage time, his offensive rating is 128.4 which is actually above the Celtics' average, mainly due to a gigangtic increase in offensive rebounding (
https://cleaningtheglass.com/stats/player/5250/onoff#tab-team_efficiency). I don't think he is an above average offensive player and that on/off is more a product of who's out there with him but there's nothing to suggest the Celtics offense falls off a cliff when he is in the game.