Well, someone will have to be second string FS. Are you saying its more likely they shuffle a cornerback in there and have Manning start at 155 lbs? I'm just confused how we can afford to redshirt any safeties.
I think that would be the plan. If Bonner got hurt (probably worst case scenario) my guess is Exum would move to FS, Fuller would move to boundary corner, and Manning would play field corner. Riley also has a chance to beat Manning for the 3rd corner spot because his body is ready.
If we had to play a freshman at safety it would be rover, because no true freshman is going to understand the defense well enough to make the right defensive calls, and my money would be on McKinnon, or maybe Riley as I think he ends up at FS eventually.