First, I think he still wants to play and unless we hear otherwise, I don't think we should try and deny him playing more when he's at the tail end of a Hall of Fame career and still playing at a quality level.
If he retired, I think you could probe him about it, but I doubt highly that he would want to coach. At least not the Celtics. The man could hardly stand sitting with the team when he was injured. I think he would just want to play too much.
At the most, he can either ride off into the sunset as mentioned or maybe I could see him mentoring kids like High School or something like that. But with the Celtics in any capacity? Shoot, he almost left with Doc. It seems like a HUGE pipe dream to me.
Let the man finish his career as he wants. If he wants to coach, I'm sure he'll make it known.