My Take 1: I think they will sign Kornet although that is not 100%. He can sign a multiyear contract and can be a useful player in 2026 and beyond (when it starts to matter again). But I doubt they go much above $6M. There may be a few other teams that can offer more than that, but I don't think too many.
My Take 2: I don't expect Horford to come back. It really doesn't make sense for Horford or the Celtics. Unless he just wants one more season as a victory lap or something but I doubt that. Most likely he signs with a contender for another run at a title. It is probably more likely than not that this is his last season.
So if our centers are Kornet, Queta, and Rookie from KY, that is pretty bleak. Our PF rotation is even more bleak; Tillman, Niang?, ????. But this is only for the lost season. There may be a trade this season with Niang (won't get much) or Simons (could return a decent big) but I don't think they want to spend money on a big this season. And I don't think they trade Hauser for a big for this season, maybe next.