This is good stuff. Thanks for the link.
A lot of this seems teachable to me. It looks like he has two major flaws.
He's slow to make a decision when he gets the ball. This allows the defense time to close out to jumpshots that they really shouldn't be able to bother, and it also allows them time to reset when he catches and holds before driving or passing. This is a habit that the Celtics will need to break early, but I don't see any reason why they can't do it.
He's not explosive around the basket. He looks fairly quick on the floor - not elite, but solid - but he really doesn't have much elevation in his game, either on his jumpshot or his finishing around the basket. Improved footwork and lower body strength will do a lot for him I think, and his length will also help, but this is always going to be a weakness.