The bad defensive numbers this year are probably just a result of the team being worse at defense in general. I find it hard to believe that his defense just randomly fell off a cliff in his third year in a league.
That said, I think it's fair to assume at this point that his realistic ceiling is a playmaking Tony Allen, assuming he can learn to play within his offensive limitations. (If not, he's an inefficient playmaking Tony Allen.) He's not going to be a superstar, but that's a fine player to have.