Amir Johnson is simply not an NBA player at this point. Have you seen his effort over the last three games? He doesn't box out, go for rebounds that don't come near him and has no value offensively. He was supposed to be effective in the pick and roll, but I have seen little of that effectiveness this season.
Zeller, on the other hand, is a much stronger, bigger body and has a much better touch both from 15 feet and around the basket.
I groaned during that sequence last night in the first half when the Knicks got six offensive rebounds. Johnson and Jerebko looked completely impotent, as the Knicks pushed them around unmercifully.
I think that this year Johnson has been exposed for the weak, ineffective player that he is.