Mitchell didn't practice Wednesday due to an illness, Matt Barrows of The Athletic reports.
The 49ers already have ruled out top running back Christian McCaffrey for Sunday's regular-season finale against the Rams due to a strained right calf, which would give Mitchell an opportunity to be the team's No. 1 back under normal circumstances Week 18. But with an illness in tow, it can't be counted on. Mitchell has two more chances this week to log on-field work, but in the meantime Jordan Mason and practice-squad member Tyrion Davis-Price represent the healthy options at the position on San Francisco's roster.