How good is Drummond, really?
He's an amazing rebounder, but his finishing numbers are poor. Some of that is shot selection / usage. He's Tyson Chandler in the restricted area; he can't score from anywhere else.
He can't shoot free throws at all. He doesn't get to the line very much for a player of his size and skillset.
Defensively, his steals are good, his block rate adequate. I've heard mixed things about what the numbers say about his defensive impact as a rim protector.
I think Drummond is elite at some of the things he does well (rebound, finish at the basket, defend the interior), and pretty poor at the things he doesn't do well (shoot the ball from any distance, pass). The key to getting the most out of him is to make it so that he's put in situations where his strengths are being utilized more than his weaknesses attacked, because his strengths can be dominant.
What I don't know is if the Celtics have the personnel to help enable this in a way that doesn't take too much away from what the team has already been doing well. He's the best rebounder in the game, and I'm not sure there's a close second. If he can be worked into the offense despite his limitations there, he very much could be a championship piece (depending on the price paid to acquire him.)