San Francisco 49ers running back Raheem Mostert Cary Edmondson-USA TODAY Sports

Unfortunately, Raheem Mostert is dealing with another significant health issue. The 49ers’ talented but oft-injured running back has chipped cartilage in his knee, head coach Kyle Shanahan announced Monday.

Shanahan said the knee issue will sideline Mostert for about eight weeks. He’ll have a scope on the knee and go on injured reserve for the time being. Elijah Mitchell, who became a workhorse against the Lions after Mostert went down early, figures to become the team’s new lead back in his place. Mitchell turned 19 carries into 104 yards and a touchdown against Detroit.

The injury likely also means fourth-round rookie Trey Sermon, who was a surprise inactive against the Lions, will get a crack at the first action of his pro career. Mostert was a 2015 undrafted free agent from Purdue who spent time with six different teams in less than two seasons before finding a permanent home with the 49ers.

He signed with San Francisco’s practice squad in November 2016 and has been with the team ever since. Impressively, he’s averaged 5.7 yards per rush on 284 career carries. Last year he had 521 yards in only eight games as he dealt with various ailments.

It’s an especially tough blow for Mostert since he’s set to become an unrestricted free agent at the end of the year. Hopefully he can return and have a lot of success in the second half to help him cash in next spring. Mostert briefly requested a trade in the summer of 2020 before he and the team patched things up with a revised contract.

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