Mostert carried the ball four times for 11 yards but also lost a fumble in Thursday's 41-39 loss to the Rams.
Mostert received his first offensive action of the season after Carlos Hyde went down with a hip injury in the first quarter. The 25-year-old suffered an untimely fumble as the 49ers tried to climb out of an early hole, and he didn't receive another carry for the rest of the game. Mostert's faux pas combined with Hyde's triumphant return could limit the speedy back to a special teams role going forward.