The San Francisco 49ers will be without their top two running backs for the foreseeable future.
Starter Christian McCaffrey will be placed on injured reserve due to a PCL injury in his right knee and top backup Jordan Mason is headed to IR with a high ankle sprain, coach Kyle Shanahan announced Monday.
McCaffrey's injury carries an expected six-week recovery period. He will not need surgery, Shanahan said.
Rookie Isaac Guerendo will now assume the starting duties for the 49ers, who dropped into last place in the NFC West at 5-7 after their loss Sunday night to the Buffalo Bills.