Mostert (knee) was placed on injured reserve by the 49ers on Monday, Cam Inman of The San Jose Mercury News reports.
Mostert had been nursing a shoulder injury heading into Sunday's game against Seattle, but the knee injury sustained in the game is what ends his 2017 season. The move leaves the 49ers without a third running back for the time being, while Trent Taylor will likely step in to Mostert's prior role on kick returns.