I'm taking Pierce side of this, Nowitzki was never very agile to begin with but since he's come back he just looks stiff. He's not nabbing boards and he's barely being used, which is compounded by his inability to find his shot... Will he get better, yes he has to.
However he won't surpass Pierce for the end of the season. With Rondo out they're going to have to find their own shot. Pierce has been doing that for years. Expect his Efficiency to go down but his scoring, FTs, and assists to go up. I expect more of the office to run through pierce now, if not by design then by teammates passing out of bad looks and him creating one. Only two big scoring threats on BOS now and Garnet is going to be seeing that pick and roll as much.