It's unlikely Ben can improve his skills and become mentally stronger to be in demand among NBA teams. In fact, he ruined his own career in the NBA. Looking at his stats last season, I think he will play a couple more seasons for minimum salary and then finish his career in the NBA.
There is not a lot to expect for Ben anymore. His career is almost and any contract that he's getting in the league should be taken by him because he knows that he's no longer the same as before. He's still fortunate that there are teams that are signing him to play for them. Overall, I don't see how he will fit with the Celtics, but like what I said, they can just waive him anytime... or who knows. They might put him on the G-league. 
Maybe that's the plan after all.