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Mitchell will have surgery on his hamstring after being placed on injured reserve Tuesday, Nick Wagoner of ESPN.com reports.

Out for most of the summer with a hamstring injury, Mitchell returned to practice Aug. 21 but then suffered a more serious hamstring injury not long thereafter. He's now out for the season and scheduled for surgery, leaving Jordan Mason as the presumed No. 2 running back ahead of a Week 1 matchup with the Jets. The 49ers hope to get Christian McCaffrey (calf) back in practice soon, and their backfield depth suddenly looks shaky with Isaac Guerendo bothered by a groin injury and with Cody Schrader having been claimed off waivers by the division-rival Rams. Mason and Patrick Taylor are the only fully healthy backs on the roster or practice squad.

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