Mostert (knee) was ruled out for Sunday's game against the Giants, Nick Wagoner of ESPN.com reports.
Mostert missed the first two days of practice this week after suffering an MCL sprain during last Sunday's win over the Jets, so it was only a matter of time before he was ruled out by the team. Jerick McKinnon and Jeff Wilson are poised to lead the 49ers' backfield for the near future, as Tevin Coleman (knee) also is sidelined.