The 49ers placed Mitchell (undisclosed) on injured reserve Tuesday.
Mitchell kicked off August with a strained hamstring that sidelined him for a good portion of training camp and the entire preseason slate, but he returned to practice last Wednesday, signaling he was making progress in his recovery. It's unclear if the aforementioned health concern precipitated this move, but he'll nonetheless miss the entire season. With Mitchell on IR, Jordan Mason, Isaac Guerendo and Patrick Taylor will be the reserve running backs behind Christian McCaffrey (calf) to begin the campaign.