Mitchell's teammate, Raheem Mostert, is expected to miss about eight weeks with a knee injury, Ian Rapoport of NFL Network reports.
Mostert exited Sunday's season opener after logging just two carries, and upon his exit Mitchell rushed 19 times for 104 yards and a TD. It was a strong enough performance to put him in the driver's seat to start this coming weekend's game against the Eagles, though it's possible rookie Trey Sermon -- who was a surprising healthy scratch Week 1 -- could re-enter the 49ers' backfield equation in short order, with JaMycal Hasty also in the mix.