If the question is fixable for 2025-26, I say no it isn't and it doesn't matter anyway. There is no reason to try and fix this season. There is no way you can replace Tatum. None. This season is what it is without Tatum.
For 2026-27, we get Tatum back and in my mind, that will "fix" most of the problems but not all of them. I feel like we would still be one good starting big away from being able to be a top team again (not an all star, just a legitimate 4th or 5th starter level player). I don't think that starting big is currently on the team, it is going to require an acquisition to fix that. Could be a FA, could be a trade.
I think it's going to require 2 new bigs, not just 1. The goal would be to remove Tillman completely from the roster and wait out Garza's contract to expire after teh 26-27 season (if he can't be traded) and get 2 legit bigs, one having to be a starting-quality center, to take their minutes. having Tatum at PF next year will help but even if we keep Boucher who's been serviceable, it's still a big talent dropoff with Tatum would hit the bench. Q should be fine as the primary back up center.
thing is, it's not just fixing the big rotation for next year but getting another PG. White and PP aren't enough. Simons isn't a PG or much of a distributor/facilitator. trading Jrue left us with a real hole at that position as well that PP isn't filling as a starter.
overall, looking at what we have now and seeing what could carry over to next season, the bench wouldn't be horrible but the problem this year is the bench players are needed for big minutes. a bench of PP, Hauser, Minott, Boucher, Q and eventually Hugo is pretty solid. we just really need a starting PG (or combo guard), starting C and another good big man (a 4/5 ideally) added to the roster. maybe that lottery pick we seem destined for this year will fill one of those holes but Brad's got to make the most of Simons as a trade asset to fill another hole at the deadline to prep for next year.