Niners coach Kyle Shanahan noted doctors are "pretty optimistic" Mitchell will be fine, adding there is hope that the running back could even practice Wednesday.
Mitchell leads the Niners with 560 rushing yards and three touchdowns on 116 carries in seven games played. The sixth-round pick leads all rookies with 80.0 rush yards per game this season.
Mitchell took 27 carries for 91 yards in Monday night's 31-10 blowout win over the Los Angeles Rams. Shanahan said it wasn't clear when the back suffered the injury, and Mitchell made no mention of an issue until Tuesday morning.
If Mitchell can't play this week, Jeff Wilson, who saw his first action of the season in Week 10 after returning from an ankle injury, would likely be in line for an increased workload. With JaMycal Hasty missing Week 10 due to an ankle injury, a Mitchell absence could also mean rookie Trey Sermon might see the field after not playing an offensive snap since Week 5.