So at the moment, we potentially have the following guys on the roster (highly subject to change):
PG: Boozer, Rippey
CG: Foster
SG: Evans
SF: Sarr, Howard
PF: Williams, Wilkins
C: Ngongba, Meyer, Iffy
The bolded guys could choose to stay or go to the draft (note: I excluded Cam, because we've not had a projected top-5 pick choose to return to college since Jay Williams did so 25 years ago). The italicized guys could stay or transfer.
That group, with the addition of a backup grad transfer big, would be a very good team. But we definitely need that backup big.
If Evans, Sarr, and/or Ngongba go pro, then Scheyer will need to replace them with somewhat of a like-for-like player. In other words, I'd expect Duke to land a starting SG if Evans leaves, a starting SF if Sarr leaves, and a starting C if Ngongba leaves. So for the purposes of discussion, I'm going to refer to those positions as Evans/Sarr/Ngongba, with the understanding that whomever goes will likely be replaced by a similar quality of player.
So that leaves the remaining four potential returnees. Iffy probably wouldn't be replaced if he transfers. Meyer would fill that role already. If Wilkins transfers, we would probably need another backup PF. If one of Boozer or Foster transfers, we probably don't need to add another guard. If BOTH of Boozer and Foster transfer, we'd need a starting-caliber guard.
I'm not terribly concerned about our ability to replace Evans and Sarr. There are going to be enough SG/SFs in the portal that we can get a star guard. Ngongba is a bit trickier a proposition, in that there are fewer elite big men to go around. That's the one I'm watching most closely at the moment.