It is a basic matter of math: the central point that you'll be seeing our 3rd-string center, Semih, a fair amount this season.
Snaq and JO's injury history indicates that Semih will be the C's backup C for 25-41 games this year. Get used to it.
The good part is that Semih is a promising project. Is he Snaq? No. But he was the core C on the silver-medal team at the FIBA world championships, and to think that he isn't a promising rookie is crazy.
I think you are overstating things. Chances are they will shorten the rotation for much of the time if Shaq or JO are hurt, with most of the minutes going to KG, Baby, and whoever the healthy guy is. And of course Harangody also will have a great shot at those spare minutes.
Erden will likely get a shot at some point this season, and he will hopefully hold his own. But let's not get out of hand here.
Also, I don't know what you mean by "the core C", but he was the backup Center on that team behind Chicago's new backup Center.