SANTA CLARA, Calif. (AP) — San Francisco 49ers running back Jerick McKinnon has hurt his knee in practice and could be facing a serious injury.

Coach Kyle Shanahan said Saturday that McKinnon got hurt in his first day of team drills since injuring his right calf Aug. 9. McKinnon will undergo an MRI and Shanahan said the team is "nervous" about his outlook.

The 49ers were counting heavily on McKinnon this season after signing him to a four-year, $30 million contract in free agency. The 49ers now have three healthy running backs on the roster in Matt Breida, Alfred Morris and Raheem Mostert.

McKinnon spent four years mostly as a backup in Minnesota. He has rushed for 1,918 yards with an average of 4.0 yards per carry, has 142 catches for 984 yards, and has scored 12 touchdowns.

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