Realistically the path to contention is something like this:
1) Tatum comes back close to 100% of what he was before the injury, in other words somewhere in the 5-8 range of NBA players.
-I feel pretty good about this one.
2) Some combo of Hugo/Walsh/Scheierman/Minott step up to become at least rotation players. I'd improve Queta on this list, but honestly I'm already sold that he's at least a backup big, maybe better.
-Minott looking great, Hugo looking promising, Walsh/Scheierman have shown flashes.
3) The Celtics use Anfernee Simons salary slot to get another big man.
So the question really is how do they do #3? They actually have a few paths. Right now they have a 22.5 million dollar TPE from the Porzingis trade, if they were to simply let Simons expire they could use that TPE to bring back a center making less than that amount. Some options include:
Zubac (19.55), Poeltl (19.5), Carter Jr (18.1), Gafford (17.3), Aldama (17.1), Onkongwu (16.1), Stewart (15.0).
Now of course some of those won't be available but depending on how their teams play this year a few of those guys probably will be. They could also "expand" the TPE by trading Simons and taking back salary into the Porzingis TPE. That would generate a new TPE for the value of Simon's contract, about 27.7 million. If they did that they could add:
Turner (26.6), Reid (23.3) and Claxton (23.3) to the list of targets.
They could also use the TPE in a S&T. That would open up free agents options such as:
Porzingis, Horford, Robinson, Nurkic, Kessler, Hartenstein, Vucevic, Mark Williams, Rob Williams amongst others.
Some of those guys (namely Porzingis) may be out of the TPE price range, but the others are probably gettable. If they don't use the TPE's they'd be far enough under the 1st apron to use probably all of the non-tac mid-level, that's a lower number (starts around 16 million) than the TPE but you don't require cooperation from another team to use it.
They have assets left to trade. At the draft they could package their 2026 1st, 2027 1st, 2031 1st, 2033 1st, 2030 swap, 2032 swap. So they would have 4 tradeable firsts and 2 tradeable swaps. More than enough to get a solid starting center.
This is why I just don't think things are as dire as some people seem to. Alot of their issues are solved a healthy Tatum and one more solid big man, and a solid big man should be pretty easily achievable.