49ers' Mostert to undergo season-ending knee surgery

San Francisco 49ers running back Raheem Mostert, who was initially expected to miss eight weeks with a knee injury, will undergo season-ending surgery, he announced Tuesday.

Mostert was diagnosed with torn knee cartilage after leaving the season opener versus the Detroit Lions. He took two carries for 20 yards before exiting.

Mostert, who tallied 772 yards and eight touchdowns on the ground in 2019 when the 49ers went to the Super Bowl, has now had two consecutive seasons cut short by injuries. An MCL sprain and a high ankle sprain limited him to eight appearances a year ago.

The 49ers featured Elijah Mitchell in the backfield after Mostert went down against the Lions, and he responded with more than 100 yards and a touchdown. San Francisco also has JaMycal Hasty and rookie Trey Sermon on the depth chart and expects Jeff Wilson Jr. to return from a knee injury at some point this season.

Mostert is due to become a free agent at the end of the year. He's been with the Niners since 2016 after beginning his career bouncing between teams as an undrafted player out of Purdue in 2015.

With Mostert on the shelf, the 49ers are signing former Lions second-round pick Kerryon Johnson to the practice squad, his agent told ESPN's Adam Schefter.