Mostert announced Tuesday via his personal Twitter account that he will undergo season-ending knee surgery.
Though 49ers head coach Kyle Shanahan estimated Monday that Mostert would require an arthroscopic procedure and miss about eight weeks after sustaining chipped cartilage in his knee in the team's season-opening win over the Lions, the running back is instead electing to undergo what's believed to be a full meniscus repair, per Mike Garafolo of NFL Network. While the surgery will necessitate a longer recovery timeline, Mostert should have a brighter long-term health outlook than if he had returned in the second half of the season and continued to play through the knee issue. Expect the 49ers to officially place Mostert on season-ending injured reserve later this week, paving the way for Elijah Mitchell, Trey Sermon and JaMycal Hasty to lead the ground game.