Mostert suffered a concussion during Saturday's preseason game versus the Cowboys, Matt Maiocco of NBC Sports Bay Area reports.
This is yet another health concern for the 49ers backfield, whose injured includes Jerick McKinnon (knee) and Jeff Wilson (calf), while Matt Breida tended to a partially torn pectoral during the offseason. Although Tevin Coleman, McKinnon and Breida figure to have regular roles, when healthy, a player like Mostert could make an impact when the RB room is less than 100 percent, as he did last season before breaking his forearm. For the time being, though, Mostert will be contained to the concussion protocol until his symptoms diminish and he's cleared by an independent neurologist.