Seriously though, I feel that JO will be able to play center but I see his best fit as a PF. In terms of Erden, some have suggested that he is not really a center either. I don't know that as I haven't seen him play but I can say that if he isn't a Center than why do we need him?
JO's position really depends on what other options you have at center and power forward. He would be a center next to KG and a PF next to Shaq. He's probably less able to play PF than he used to be, though. Rasheed Wallace was a starting PF next to Sabonis, but probably the starting center later on next to Dale Davis. Was he the power forward or center next to Ben Wallace in the Pistons starting lineup. By the time Sheed got to Boston, his mobility had declined enough that he seemed to be primarily a center. A young Jermaine O'Neal's best fit was clearly PF. An older JO, maybe not.
If Erden can develop, he may be one of those 7-foot-ish PF-C backup bigs. Tall enough to guard some centers, athletic enough to guard some power forwards, but not good enough to be a starter at either position. He might be useful against a big who doesn't post up much but can shoot over a guy like Glen Davis. (Imagine someone a bit Sheed-like.)
If Erden could be a lengthy athletic big with some energy who blocks shots decently, rebounds at a mediocre but not atrocious level, and is better on put-backs than Perkins or Big Baby and better at pick and roll defense than Shaq, he'd be a useful situational player even if he does have a bit of a foul problem. That is certainly something within his range of outcomes.
I think it is pretty clear that Erden has the physical tools to make the regular rotation of an NBA team and the main questions revolve around his ability to learn and think on the court. He should be at least mildly useful if he can learn and execute team defense.
