Coach Kyle Shanahan said Mostert (ankle) won't practice Wednesday, Cam Inman of The San Jose Mercury News reports.
Mostert has been bit with the injury bug for a second time this season, this time tending to a high left ankle sprain in the aftermath of Sunday's win over the Rams. Shanahan told Nick Wagoner of ESPN.com on Monday that Mostert will "most likely" be placed on injured reserve, but the 49ers have yet to make the transaction official. An IR move would sideline Mostert for at least three games, and with Tevin Coleman (knee) unlikely to be activated from IR this week, San Francisco will again be shorthanded in the backfield. Jerick McKinnon, Jeff Wilson (calf) and JaMycal Hasty are expected to serve as the 49ers' backfield options Sunday in New England.