The fourth-year running back suffered a broken right arm in Thursday’s 34-3 rout of the Oakland Raiders and likely is out for the season. He exited the field wearing an air cast.
Mostert was injured at the end of the run in the third quarter when his right arm bend back awkwardly after he planted his hand to break his fall.
San Francisco coach Kyle Shanahan confirmed the injury to reporters after the game.
“They didn’t tell me, but I assume (his season is over),” Shanahan said, per NBC Bay Area Sports.
Mostert was on the way to the best game of his career, rushing for 86 yards on only seven carries and scoring his first NFL touchdown on a 52-yard run in the third quarter.
The 5-foot-9, 197-pound Mostert was a yard shy of his season high set against Green Bay on Oct. 15, when he had a career-best 12 rushing attempts.
An undrafted free agent from Purdue in 2015, Mostert entered this season with only seven total carries — six coming with the 49ers a year ago.
A season-ending torn ACL to Jerick McKinnon in the preseason and injuries to backup Matt Breida opened the door for more playing time for Mostert, who has 261 yards rushing on 34 attempts, averaging 7.7 yards per carry. He also had six receptions for 25 yards.
With Mostert sidelined, the 49ers will have to lean on Breida and veteran Alfred Morris. Breida had 44 yards on 12 carries in Thursday’s victory while Morris picked up 13 yards on seven rushes.