It could be worse, we could have broken Sully who again got injured. I think Horford is a huge upgrade. Zeller had solid rebounding numbers.
He battles players who are heavier and more athletic. No matter what he does isn't going to change that.
I do wish he would hit the weights so he was not pushed around as much. That could change that for sure.
Johnson was horrible he needs to be traded with picks for a true center.
Amir plays solid D, most of the time, he does look long in the tooth. If he was as bad as you say who would take him, isn't that fantasy. I think he looks better when KO returns and he has to play less minutes.