Coach Kyle Shanahan relayed that Mitchell (knee) didn't practice Friday due to an illness, Matt Barrows of The Athletic reports.
Shanahan hinted Monday that a return to practice for Mitchell was possible before week's end, which would have opened his 21-day window to return from injured reserve. Instead, Mitchell has caught the bug that's impacted a few 49ers this week, thus delaying his first on-field work since spraining his MCL on Nov. 27. Assuming he's designated for return from IR next week, he'll have a chance to play Week 18 against the Cardinals, but more realistically the team is aiming to have him available for the postseason as a backfield option behind Christian McCaffrey.