Running back Raheem Mostert is expected to miss half the season after sustaining a knee injury early in the first quarter of the 49ers’ season opener Sunday against the Detroit Lions.
Coach Kyle Shanahan said Mostert “chipped off” a piece of knee cartilage. He will undergo an arthroscopic procedure.
The 49ers are anticipating that Mostert will be able to return to action in eight weeks, which would place his return some time after the Nov. 7 game against the Arizona Cardinals.
Mostert, who opened Sunday’s opener as the team’s starting running back, would miss both games against Arizona and the first meeting against the Seattle Seahawks.
Rookie running back Elijah Mitchell took over as the 49ers’ primary halfback on Sunday and gained 104 yards and a touchdown on 19 carries.
Mitchell was selected in the sixth round of the draft, while Trey Sermon, the team’s third-round pick, was inactive for the game.
Now, Sermon will join Mitchell and JaMycal Hasty as the team’s active running backs.
Running back Jeff Wilson Jr. sustained torn knee cartilage in the offseason and is expected to be ready for a midseason return, too.