Raheem Mostert announced Tuesday on Twitter that he is undergoing season-ending knee surgery.
The 49ers running back injured his knee early Sunday in the 49ers' 41-33 Week 1 win against the Detroit Lions.
Mostert opened the season as the 49ers' starting running back. He rushed for 20 yards on two carries, but lasted only four snaps on offense.
Coach Kyle Shanahan on Monday told reporters that Mostert was expected to miss half the season after chipping off a piece of knee cartilage. He was to undergo an arthroscopic procedure.
Mostert, 29, signed a three-year, $8.7 million contract with the 49ers in March of 2019. The 49ers restructured his deal following that season after Mostert broke out in the playoffs. But Mostert only lasted eight games last season, missing half the season with an ankle injury.
In those eight games, he rushed for 521 yards, averaging a career-high 65.1 yards per game.
With Trey Sermon as a surprise healthy scratch in Week 1, rookie Elijah Mitchell took over as the 49ers' lead ball-carrier against the Lions with Mostert sidelined. The sixth-round draft pick made 49ers history, rushing for 104 yards on 19 carries, including a 38-yard touchdown.
In response to Tuesday's news, the 49ers are signing former Detroit Lions running Kerryon Johnson to their practice squad, his agent told ESPN's Adam Schefter.
The 49ers now have lost their starting running back and their top cornerback, Jason Verrett, to season-ending knee injuries after one game.