Mostert (knee) is questionable to return to Sunday's game against the Lions, according to Matt Barrows of The Athletic.
This isn't a promising sign for the 49ers, as they're already quite thin in terms of depth at running back. Elijah Mitchell and JaMycal Hasty are expected to gain a bigger role if Mostert is unable to return to action in the contest, given that Trey Sermon is inactive Sunday.