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Mostert suffered a sprained MCL in Week 2 against the New York Jets and missed two weeks before returning last week against the Miami Dolphins. Despite the Niners losing to the Dolphins by a 43-17 score, Mostert rushed for 90 yards.
He entered Sunday’s game with 238 rushing yards and one touchdown on the season, as well as nine receptions for 139 yards and one score.
Before exiting against the Rams, Mostert had rushed for 65 yards on 17 carries and caught two passes for 11 yards.
With backup running back Tevin Coleman on the shelf because of a knee injury, Jerick McKinnon was the only other halfback who was active for the Niners for Sunday’s game.
McKinnon, who missed the entire 2018 and 2019 seasons, has been a quality performer for the 49ers this season, as he entered Sunday’s game with 193 yards and three touchdowns on the ground, plus 15 catches for 107 yards and another touchdown.
If Mostert misses more time moving forward, McKinnon will likely serve as the No. 1 back for head coach Kyle Shanahan.