Mitchell will have surgery on his hamstring after being placed on injured reserve Tuesday, ESPN.com's Nick Wagoner reports.
The 49ers placed Mitchell (undisclosed) on injured reserve Tuesday.
Mitchell (hamstring) resumed practicing Wednesday, Nick Wagoner of ESPN.com reports.
Mitchell (hamstring) is not expected to play in Saturday's preseason opener, Matt Maiocco of NBC Sports Bay Areareports.
Coach Kyle Shanahan relayed Tuesday that Mitchell is dealing with a strained hamstring, Lindsey Pallares of the 49ers' official site reports.
Jordan Mason may challenge Mitchell for the No. 2 RB role, Jordan Elliott of NBC Sports Bay Area reports.
Mitchell remains the clear No. 2 running back for San Francisco, David Lombardi of The Athletic reports.
Mitchell had four carries for seven yards and a touchdown Sunday in the 49ers' 34-31 win over the Lions in the NFC Championship Game.
Mitchell rushed for 52 yards and a touchdown on 14 attempts while catching both of his targets for 15 yards in Sunday's 21-20 loss to the Rams.
Mitchell (illness) doesn't have a game designation for Sunday against the Rams, Eric Branch of the San Francisco Chronicle reports.