Jacobs was assisted to the sideline, unable to put weight on his left leg. He later was carted away to the 49ers' locker room.
The 49ers' deep backfield took another hit when rookie LaMichael James was carted off with a left foot injury in the fourth quarter, which isn't believed to be serious, according to The Sacramento Bee. He finished the night with four carries for 19 yards. Losing a pair of backs would cripple some teams, but San Francisco will lean on Gore and Hunter in the interim.
UPDATE: Coach Jim Harbaugh confirmed Jacobs suffered no damage to his anterior cruciate ligament. He told the team's official website the veteran "will miss some time," but suggested Jacobs would be back this season.
NFL.com and NFL Network's Ian Rapoport reported Jacobs will have an MRI on Sunday to determine the extent of the injury. A source told Rapoport that Jacobs doesn't appear to have suffered major structural damage.