Coach Kyle Shanahan told Nick Wagoner of ESPN.com that Mostert has been diagnosed with a high left ankle sprain and "most likely" will be placed on injured reserve.
Mostert sat out Weeks 3 and 4 with a sprained MCL and now is poised to miss at least three more games due to the ankle issue that forced him out of Sunday's win against the Rams. Shanahan also mentioned Tevin Coleman (knee) is a "long shot" to be activated from IR this week, per Matt Barrows of The Athletic, leaving Jerick McKinnon, JaMycal Hasty and Jeff Wilson (calf) as the likely running backs available to the 49ers offense this coming Sunday at New England.