Mitchell (knee) was placed on injured reserve Saturday, Adam Schefter of ESPN reports.
It's important to note that Mitchell could, and likely will, return during the 49ers' postseason run if the team gets to the playoffs, but this designation confirms the second-year back is out through at least Week 16. Based on the initial timetable placed by coach Kyle Shanahan earlier in the week, the postseason appears a more likely target for Mitchell's return to action. The 49ers did not choose to call up Tevin Coleman from the practice squad, meaning Jordan Mason and Tyrion Davis-Price will be left to back up star running back Christian McCaffrey on Sunday against the Dolphins.