I like Sully, he's a special rebounder, a good passer, and has a solid basketball IQ, but he is not a max player. I would be in favor of signing him to a team friendly 3 year contract. Mostly because of his weight, injury and conditioning issues.
Sully has yet to prove he is able to cover most of the leagues power forwards on defense, he is covering the opposing teams center, using his bulk to control them from setting up in the paint. That would label him a center. Is he a starting center on a seriously contending team? That's the question the Celtics need to answer going forward. If Sully is your center, you should look for a stud power forward, with length, rim protecting ability, and solid scoring to pair with him.
If you say no, Sully isn't my choice as the starting center, then why give max money to what will end up a backup center.