Coach Kyle Shanahan said after Sunday's 27-13 loss to the Saints that McKinnon suffered a stinger late in the game, Jennifer Lee Chan of NBC Sports reports.
Prior to the injury, McKinnon rushed 18 times for 33 yards and caught one of three targets for 13 yards. Fellow running back JaMycal Hasty left earlier in the contest with a collarbone injury, but San Francisco will have the benefit of a bye next week to allow the team's injury-riddled backfield to heal up.