Mostert missed Wednesday's practice due to a knee injury, Nick Wagoner of ESPN.com reports.
The extent of the knee injury isn't known, but coach Kyle Shanahan termed it a "sprain" or "strain," per Matt Maiocco of NBC Sports Bay Area. With Mostert's status in question for Sunday's contest at Washington, the door may be open for Jeff Wilson to rejoin the 49ers' running back rotation alongside Matt Breida and Tevin Coleman.