Coach Kyle Shanahan said Mostert (ankle) won't practice Wednesday, Nick Wagoner of ESPN.com reports.
Mostert has missed four games this season due to a high-ankle sprain, but he's also served in a timeshare at running back for each of the past three contests, racking up 156 yards from scrimmage and one touchdown on 44 touches during that span. Shanahan told Wagoner on Monday that Mostert emerged from this past Sunday's loss to Washington with an ankle issue that required "further imaging." The results of those tests haven't been revealed, but Mostert isn't exactly kicking off Week 15 prep in great shape. Jeff Wilson, Tevin Coleman and Jerick McKinnon are the next running backs up for the 49ers.