Mostert (knee) has been ruled out for the remainder of Sunday's game against the Lions, Keiana Martin of the 49ers' official site reports.
The knee injury Mostert suffered in the first half of Sunday's game will prevent him from seeing any more action in Week 1, so Elijah Mitchell and JaMycal Hasty will split the carries for the remainder of the day. The extent of Mostert's injury is not yet known, but head coach Kyle Shanahan figures to provide some sort of update after the game concludes.