I really don't see a problem here. There's likely minutes for Johnson, Zeller, Sullinger, and Olynyk, though of course we'll have to see who is still on the roster come opening night.
I also don't get this whole "Johnson isn't a center" argument. What exactly makes Zeller a center and Johnson not one? Zeller is 7-0, 250 with short arms, little post game, and little shot blocking ability. Johnson is 6-9, 240, with better shot blocking. I see no reason why Johnson can't play center when Zeller did just fine there last year. And if BCS is going to play lineups with Bass/Sully at the 4/5 and at times Crowder/Bass, you can bet Johnson will see his fair share of action at the 5.