Mostert didn't play in Saturday's preseason game at Kansas City due to a minor quadriceps injury, Eric Branch of the San Francisco Chronicle reports.
Mostert already missed some time during training camp with a concussion, but this marks his first DNP of exhibition season. While just Jerick McKinnon (knee) had a known health concern before this contest, the 49ers backfield has been in regular flux in both of coach Kyle Shanahan's seasons to date. Mostert will look to shed the injury before the team's preseason finale Thursday versus the Chargers.