On one hand, I think this is kind of a non-issue this year. Even if the C's surprise and make the playoffs, they aren't going to contend seriously, so if they don't have a true center this year, it isn't that big of a deal. But it certainly should be a long term goal.
And while I don't hate Tyler Zeller, I don't get why some people are acting like he's any more of a center than Sully and Olynyk when you factor in the fact he has short arms, doesn't block shots, and doesn't have much of a post game. He might prove to be a decent enough bench big man, but right now I think we're better off playing Sully and Olynyk next to each other as much as possible.