While it'd be nice to upgrade, I'd be content to go into the season with KG, Wilcox, and either Hollins or Williams as the third string center.
Wilcox proved last year that he could be quite explosive around the rim (particularly when playing with Rondo). While it was probably hyperbolic to some to compare him to Shaq of '10-11, he was successful for some of the same reasons: players who can finish with authority around the rim will find success playing with Rondo (and other elite players like Pierce and KG who draw lots of defensive attention).
Now with Wilcox, we gave him the the luxury MLE last year at roughly 3 million a year. We don't have his Bird rights. However, can't we still offer him the same 3 million plus a small raise? If so, he's probably our most realistic option at this point give our financial constraints.
While I don't love the idea of Hollins or Williams playing heavy minutes, assuming the team stays healthy, having either play spot minutes is fine. Hollins also proved that he can play some serious minutes at times and be successful. Williams could also be a player who at the very least can block some shots and could potentially develop into something more under the tutelage of KG.