He does seem like a knucklehead a little bit but guys who can put up 21 / 18 nights have something are worth taking a look at.
I'm wondering if he came to a good program like the Celtics with all the Vets, Doc, Ainge, etc. and he was engrossed in a winning program, does the kid get dialed in and grow up?
If he focused just on defense and rebounding, he could be an absolute nightmare along side KG on defense.
For the right price I would 100%, without reservation go after this kid. If it doesn't work out, you will be able to move him anyway. There will always be a team that will take a flyer on a 7 foot center with good to decent skills - just look at the history of the NBA.