I wouldn't mind seeing him sign a team friendly contract, or let just let him walk.
He's an undersized back up center, he'll never be in condition to play his natural position of power forward. If you've looked closely, the listing of his weight has gone from 260 lb. (a prayer) to the real 280 lb. It's a matter of time before that foot or back problem resurfaces, he takes time off, and gets back to 300 lb.
Sooner or later, the Celtics will find a suitable starting center, and Sully could be a decent/good bench
player to bring off the bench at certain times of the game, for short minutes.