49ers head coach Kyle Shanahan told reporters that Mostert is expected to miss about eight weeks due to "chipped cartilage" in a knee. The injury requires arthroscopic surgery, and San Francisco will place the tailback on IR in the coming days.
Mostert played just four snaps on offense before exiting the 49ers' 41-33 win over the Lions. Elijah Mitchell replaced Mostert as the lead ball carrier and set a franchise record for rushing yards -- 104 yards on 19 carries -- in a debut.
Hasty, who ran for a touchdown against the Lions, is expected to serve as the backup behind Mitchell. Rookie running back Trey Sermon moves up to third on the depth chart.
"Fortunately, I think we'll get Raheem back later this year, so we have got to make sure those guys hold down the fort while he's gone," Shanahan said.
Also Monday, Shanahan confirmed that cornerback Jason Verrett sustained a season-ending ACL injury in his right knee.
Verrett, who tore his left ACL in 2016 and his right Achilles in 2018, suffered the injury late in the fourth quarter against the Lions. He was eventually helped to the locker room, and subsequent imaging revealed the extensive damage in the knee.
Shanahan said the 49ers plan to stick by Verrett as he begins another lengthy rehabilitation process.
"I'll talk to Jason about anything," Shanahan said. "It's always a tough situation, but I'll believe in Jason until he doesn't want to do it. If anybody can get through stuff like this, no matter how many times it's happened, it's Jason."