There's nothing wrong, per se.
It's some combination of being worn out from carrying the load all year and being regularly double-and-triple-teamed in the playoffs. Hopefully, Danny will get him the help he deserves and the Cs desperately need in the form of another, at least, dependable scorer.
Consider where the Celtics would be without him. You can easily make a case for him as league MVP.
I believe he is.