At this point, I rather give him a chance then T-Mac.
There is an interesting article in ESPN mag today about him. It sounds like he is ready to do things like 10 day contracts and d-league (something he wouldn't do last year) as ways to get back to the NBA.
Plus, he is good friends with KG.
The thing that got me, everyone says he is a good NBA player. He just needs to improve his body language and personal interactions with teammates and coaches on the court.
Give him an invite to summer camp. A partially guaranteed 1 year min deal. If he plays well and fits in, the Celtics have a 6th man. If not, waive him.