The Niners on Friday placed Mostert on injured reserve. He will need season-ending surgery, coach Kyle Shanahan said.
With 4:38 to go in the third quarter, Mostert took a handoff to the left side and was taken down for no gain by Raiders defensive tackle Clinton McDonald. Mostert and the Niners immediately knew something was wrong as the team's medical staff raced to his side.
A few minutes later, the team announced the injury and ruled Mostert out for the rest of the game. Before the injury, Mostert had rushed for 86 yards in seven carries, including a 52-yard touchdown.
In recent weeks, Mostert had emerged as Matt Breida's primary complement in the running game, leaving Alfred Morris as the third back. He was also considered the Niners' best special-teams player.
Mostert had 34 carries for 261 yards this season, averaging 7.7 yards per attempt.
With Mostert out, Morris figures to move back into a more prominent role, and the Niners will have to add another back to the active roster for depth.
One option is practice squad running back Jeff Wilson, an undrafted free agent the Niners signed out of North Texas in May.
Also Friday, Shanahan said that cornerbackEmmanuel Moseleywill need season-ending surgery after dislocating a shoulder against the Raiders.
Mostert runs for 52-yard TD
Niners running back Raheem Mostert turns up the sideline and goes untouched for a 52-yard touchdown.