With Horford & Kornet
G: D White, Pritchard
G: A Simons, Scheierman
F: Jaylen, Hauser
F: Horford, Niang
C: Kornet, Queta
That is a reasonably strong team team in terms of balance and depth. Lacks the high end talent to compete at the top of course but good enough to be a solid regular season team. Above .500.
Without Horford & Kornet
G: D White, Pritchard
G: A Simons, Scheierman
F: Hauser, (min contract FA)
F: Jaylen, Niang
C: Queta, (min contract FA)
Keeping Horford will be imperative. Having that 2nd big man who can help the center position is needed since the center rotation will be one of the worst in the league. Without Horford, we'll have to play small and rely on only one big to control the paint (Kornet, Queta). And that one big will be among the weakest in the league. So we must keep Horford to make the interior defense & rebounding work.
If Horford does leave, I believe starting Hauser will be the best option likely with Jaylen having to defend the PF position. Niang is too poor a player to start. He is fine 15mpg backup PF but he can't be playing more than that. We have no other big forwards without Tatum. It will be hard to find one as a minimum contract FA who is good enough to play 30mpg. So it will likely fall on Jaylen as the next biggest (bulk, strength) wing player we have to play up a position and defend the PF position.