Running back Jerick McKinnon tore his ACL in practice Saturday, a source informed of the situation told NFL Network Insider Ian Rapoport.
McKinnon was expected to play a large role in his first season with the 49ers after showcasing his dual-threat pass-catching abilities in Minnesota last year. He signed a four-year, $30 million deal with the team in March.
Niners coach Kyle Shanahan described what happened to McKinnon while speaking to reporters after Saturday's practice session.
"He made a cut on air. It was the last play of practice," Shanahan said. "He made a cut on air, no one was around him. He just went down. It looked awkward."
Alas, injuries have already curtailed his availability. The 26-year-old was just coming back from a muscle strain in his calf.
"Today was the first day we allowed him to practice with the guys," Shanahan said. "On the day of the game he was out there running routes, doing stuff with the other guys not dressing with the quarterbacks, and things like that. He's been running routes and doing things but today was his first day back in live drills, or 11-on-11 drills."
The 49ers' backfield is also dealing with an injury to Matt Breida (shoulder). The injuries, along with the releases of RBs Jeremy McNichols, Jeff Wilson Jr. and Joe Williams, put veteran Alfred Morris on track to play a big role early in the season.