Fozzy is the backup and would get the starting opportunity, with CAP being the change of pace and possibly splitting carries. I'd say 60/40 Fozzy at best. Tolbert's role likely wouldn't change too much...he'd maybe get a couple extra carries.
With that said, there doesn't appear to be too much concern about Stewart or Tolbert missing this week. Both guys routinely take Wednesday and sometimes Thursday off. I can't recall the last time Stewart practiced on a Wednesday.
I do think there is a possibility that if Stewart were to miss a game, he may lose the starting job. Last week, he was out for a series and the offense was immediately better with Fozzy as the RB.