Before the injury, Raheem Mostert recorded his first career touchdown.
Head coach Kyle Shanahan said Mostert broke his arm.
The 26-year old was injured in the third quarter, as he landed awkwardly and the arm appeared to bend.
Before the injury, Mostert recorded his first career touchdown.
Mostert recorded seven carries for 86 yards before he exited the game to lead the 49ers running back corps.
He has played in eight games this season, but his role increased starting in Week 6. Mostert recorded 12 carries for 87 yards against the Packers and added 59 yards against the Rams on Oct. 21.