"It wasn't even a full practice," Rapoport reported on NFL Network. "It didn't really seem like there was much there but he suffered a non-contact injury just making a cut. Went down. It was very clear to the 49ers this was serious."
"He made a cut on air," head coach Kyle Shanahan told reporters. "It was the last play of practice. Made a cut on air. No one was around him. Just went down. It looked awkward."
The 49ers later confirmed that McKinnon tore his ACL and would be undergoing season-ending surgery.
"Right off the bat, I am told [the 49ers] are crushed by this injury," Rapoport continued.
McKinnon had not played since the 49ers exhibition opener on August 9. Three days later, he sustained a calf strain during practice and was not slated to play again until the regular season opener against the Minnesota Vikings.
Shanahan said that Saturday's practice was McKinnon's first time participating in 11-on-11 drills since the calf injury.
The 49ers signed McKinnon in March to a four-year deal worth up to $30 million. He was intended to become the team's featured back within Shanahan's offense.
The Vikings made McKinnon a third-round draft pick in 2014. He registered 1,918 yards on 474 carries with seven touchdowns in four NFL seasons. McKinnon also added 94 receptions for 676 yards with four touchdown catches since 2016.
49ers defensive coordinator Robert Saleh is well aware of the matchup problems McKinnon created for opposing defenses. Saleh saw the running back as a dangerous receiving option for the offense, almost like another wide receiver.
"A wide receiver who can carry the ball and carry it with power the way he does," Saleh said in August. "He's another dynamic football player that's got to be accounted for."
The other running backs on the roster after Saturday's 53-man cut-down are Matt Breida, Alfred Morris, and Raheem Mostert.
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