My feeling is that if he performs well in the playoffs, he could get a decent cotract next year, which I'm sure the Celtics probably aren't willing to give him. But if he continues to play at the level he's at now (although he has been getting better), we might be able to sign him again next season.
I think with a training camp under his belt he can be a very good player with this team. I'm just not sure if he is willing to accept a backup role for yet another season, since he probably only has a few years left of playing at a high level.