Bush probably starts if Bell is down (unless Thomas is back). I'm sure Hamilton will be activated, and you could even see P.J. Hill moved to the active roster ... just not sure who they would waive (off the 53) if they did have to move Hill to the active roster.
Bush starts regardless, these are roles Bell and PT play people, if Bell is down I bet PT takes on that role in the offense, I didnt see a knee brace on him when he came in and he was on kick return duty with Roby.
Even if he was active, I don't blame Payton for not putting him in at that spot. I can understand trusting the veteran more than the kid (which is funny to say because they are both the same age, lol, but Bush has the big edge in experience).