Well, if he's moved up to #2 QB, at least he should get reps in practice now. Not too much point in giving Foles practice reps I'd say.
I'd start Keenum if he's physically ready, and figure that if it's a blowout (either way - hah!) then I'd get Mannion some snaps, as
@flv suggests.
They drafted Mannion with their 3rd - they have to have believed that he might become an ok or better starter, and should at least be an inexpensive decent #2. They need to start seeing that, without totally throwing him onto the field behind a makeshift OL.