WHITE SULPHUR SPRINGS, W.Va. -- The San Francisco 49ers picked up their first win of the season in the standings on Sunday, but it came at the cost of two important losses to the roster.

Niners coach Kyle Shanahan announced Monday that running back Raheem Mostert will miss about eight weeks because of "chipped cartilage" in a knee. The injury requires arthroscopic surgery and lands Mostert on injured reserve.

Shanahan also confirmed that additional imaging on cornerback Jason Verrett's right knee revealed a torn anterior cruciate ligament, an injury that will also require surgery and end Verrett's season less than a full game into it.

Those two injuries, combined with nearly blowing a 28-point lead against the Detroit Lions on Sunday, dampened the Niners' mood after their 41-33 victory. That Mostert and Verrett were lost for such lengthy periods after a 2020 season in which the Niners were ravaged by injuries left Shanahan answering whether he has to warn his team not to fall into a "here we go again" trap.

"That's part of football," Shanahan said. "I'm not going to sit here and be a therapist to everyone about it. I thought we got out of the game mostly healthy. It was two unfortunate things for both of them. 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."

The "those guys" to whom Shanahan is referring are rookie running backs Elijah Mitchell and Trey Sermon and second-year pro JaMycal Hasty.

Mitchell figures to get the first crack at stepping into the starting job after he set a 49ers franchise record for rushing yards (104 on 19 carries) in a debut. Hasty, who ran for a touchdown against Detroit, is expected to move up to second in the rotation with Sermon third.

Sermon was something of a surprise inactive Sunday after spending most of the preseason as the first back off the bench behind Mostert. Shanahan explained that Sermon had simply been beaten out but that the team will now look to the third-round pick to provide depth.

"He really showed some stuff in the preseason and looked good, and then Elijah came in and looked a little bit ahead of him on the limited plays that he got and no one was passing up Raheem," Shanahan said. "So, you only can get three guys up. It's not an easy decision, but that's part of the NFL. It's a matter of time. We haven't gotten through a year here where we haven't started at least four backs, and now we're in Week 2 and he's up regardless."

Jeff Wilson Jr., who was Mostert's primary backup in 2020, is working his way back from a meniscus injury he suffered in the spring. Shanahan said Wilson is "definitely on schedule" to come back, with the hope that he could return around midseason. Wilson is on the physically unable to perform list, which means he isn't eligible to return until Week 7 at the earliest.

"He's looked great around here, working out, getting ready, and I know he'll be helping us at some time this year," Shanahan said.

Verrett, meanwhile, will miss the rest of the season after suffering his injury with about 7½ minutes to go against the Lions. Verrett, who tore his left ACL in 2016 and his right Achilles in 2018, seemed to know right away what the injury was.

Despite his best attempt to walk to the sideline, Verrett stopped just short of it and reached for his injured knee again before he was helped to the locker room. Verrett and the Niners feared an ACL tear right after the game, a fear that proved true Monday.

Shanahan said he spent a lot of time with Verrett on Sunday night and Verrett was understandably upset, particularly after working so hard to bounce back from previous injuries.

For now, Verrett is planning to stay with the team while it practices at The Greenbrier resort in West Virginia leading to Sunday's game against the Philadelphia Eagles. Shanahan said the Niners are working to get Verrett's wife to the East Coast so she can be with him.

Shanahan said the 49ers intend to stick by Verrett as he embarks on another lengthy rehab.

"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."