San Francisco 49ers running back Jerick McKinnon tore his right anterior cruciate ligament in practice, the team confirmed.
McKinnon fell awkwardly after making a cut on the final play of practice on Saturday and reportedly is expected to miss the entire 2018 season.
"Yeah, I mean anytime I watch someone feel their knee, there's always that concern," 49ers coach Kyle Shanahan said before the diagnosis, hopeful of better-than-expected news.
McKinnon played four seasons with the Vikings before signing a four-year, $30m deal with the 49ers in the offseason. He rushed for a career-best 570 yards last season.
San Francisco released three rushers earlier Saturday before the injury - Joe Williams, Jeremy McNichols and Jeff Wilson Jr.
The 49ers currently have running backs Matt Breida, Alfred Morris and Raheem Mostert on the roster in addition to McKinnon. Breida has missed the past three preseason games due to a shoulder injury, but Shanahan said he will be available for the season opener against the Vikings, live on Sky Sports.