Tony's offense improved greatly, his D is better (if not the same with the only change being a lot more time to poke balls away) than last year. He never would've been that in Boston.
I think he ingeniously thought this through, maybe with Doc and Danny even. Get out of Boston, you won't get much play time here, you got an amazing talent, go develop it on a smaller team and see what first class team trades 10 players to get you the year after.
However, as he said: "I'm a Celtic, just wearing different colours right now." Really sounds like both sides know he'll be back.