Raheem Mostert says he’s out for season; 49ers sign Kerryon Johnson

San Francisco 49ers running back Raheem Mostert announced on social media Tuesday that he has opted to undergo season-ending knee surgery following a Week 1 injury.

Mostert suffered chipped knee cartilage Sunday during San Francisco’s win over the host Detroit Lions. He was placed on injured reserve Monday, with coach Kyle Shanahan saying at the time that Mostert would receive an arthroscopic procedure and miss eight weeks.

ESPN reported Tuesday that the 49ers signed free agent running back Kerryon Johnson to bolster the position.

“Faithful, After consulting with multiple doctors and weighing my options, I will be having season-ending surgery to repair my knee,” Mostert wrote Tuesday. “This gives me, without a doubt in my mind, the best possibility at coming back 110%.”

Mostert left the Week 1 contest injured after taking two carries for 20 yards. His wife later claimed that Niners fans sent her “HUNDREDS, THOUSANDS” of abusive messages on social media, angry that the injury-prone Mostert was hurt again.

San Francisco ended up riding little-known rookie Elijah Mitchell for 104 rushing yards and a touchdown on 19 carries. The depth chart also includes third-round draft pick Trey Sermon, a healthy inactive on Sunday, and JaMycal Hasty.

Johnson joins that stable of backs with three years of experience (34 games, 16 starts) with the Detroit Lions. The former second-round draft pick was waived in May and claimed by the Philadelphia Eagles the next day, but the Eagles later waived him with an injury settlement.