I think both will go. Jennings will start as cover, maybe Foakes too. But Foakes may end up starting - Buttler is in the last chance saloon at the Wanderers and that's not really the pitch you'd pick for a save your place innings.
Tbh, even if he gets 80 I'd be dropping him. It's ton or bust for me. Not so much for the runs, but has anyone seen the quality of his glovework against spin on this pitch? All series really - he's way below the standard of Foakes (and to be fair, Bairstow) in this department.
More than that, really. His turn to cement his place - the others have fucked it.