Mostert suffered a high ankle sprain in last Sunday night's win against the Los Angeles Rams. On Monday, coach Kyle Shanahan said Mostert was expected to miss some time, but the Niners waited until they made decisions about other roster movement before making the move official.
Now that he's on injured reserve, Mostert must miss the next three games before he's eligible to return, though he could be out longer. In addition to placing Mostert on IR, the 49ers activated safeties Jared Mayden and Johnathan Cyprien from the practice squad for Sunday's road game against the New England Patriots.
That means San Francisco has an open spot on the roster heading into next week, a spot that likely will be filled by linebacker Jordan Willis. A league source told ESPN's Rich Cimini that the Niners and New York Jets agreed on a trade earlier this week that sends Willis to the Bay Area. Willis must pass COVID-19 protocols before the deal can be made official. The earliest that can happen is Monday, according to league guidelines.
Without Mostert, the 49ers are expected to lean on a running back group consisting of Jerick McKinnon, JaMycal Hasty and, potentially, Jeff Wilson against New England. Wilson is listed as questionable with a calf injury, but he practiced all week, and a source told ESPN's Adam Schefter that he is expected to play.
"Jeff got more [work] as the week went," Shanahan said Friday. "Today, he got a little bit more than he had the last two days, but again, it was walk-through. So it was nice to have him out there for about 10 plays yesterday. I'm hoping this flight [to Massachusetts] will treat him right and he'll give us the green light on Sunday."
The 49ers could be getting more depth at running back next week, as Tevin Coleman continues to make progress from the knee injury that sent him to injured reserve on Sept. 23. Coleman has been working on a side field in practice this week and is nearing a return, though next week might be considered a bit ambitious.
"Tevin, outside chance [to return] next week," Shanahan said. "I'm hoping that he can come back, but it's definitely not a guarantee. We'll probably find out a little bit more later in the week next week. If not that, hopefully the week after."
Mayden and Cyprien will provide safety depth Sunday after the Niners ruled out starters Jaquiski Tartt (groin) and Jimmie Ward (quadriceps) on Friday afternoon. Marcell Harris and Tarvarius Moore are the team's only other healthy safeties.