SANTA CLARA, Calif. _ Running back Jerick McKinnon, the 49ers prized free-agent signing this year, sustained a serious knee injury in practice Saturday and the 49ers are awaiting test results to see if it involved a season-ending ligament tear, coach Kyle Shanahan said.
McKinnon has been out of team drills since straining his right calf Aug. 9. Saturday marked his first day back in team drills.
"It was the last play of practice, made a cut on air, no one was around him, just went down, looked awkward," Shanahan said. "We're concerned about it but we don't have an MRI done so we don't know for sure.
"He's going to get that done right now. Something with a knee. We'll find out later. But we're nervous about it, that's for sure."
Shanahan noted the "unusual" timing of McKinnon's injury, in that it came a couple hours before the 49ers' roster got pared to the 53-man limit prior to the 1 p.m. deadline.
"We are extremely concerned. But we don't know for sure until we get an MRI, so he's got to be a part of our 53," Shanahan said. "If we get a bad MRI tomorrow, obviously he'll have to go on (injured reserve)."
Asked if he fears it's a torn anterior cruciate ligament, Shanahan replied: "Anytime I watch someone feel their knee, that's always a concern. But I felt that way before and it hasn't been. I'm not trying to be the darkest in the world. We hope that we're wrong."
Among those cut Saturday were running back Jeremy McNichols and Jeff Wilson. On Friday, the 49ers released running back Joe Williams, a 2017 fourth-round draft pick who sustained a rib fracture two weeks ago.
"We're concerned but we're also confident," general manager John Lynch said. "We've got a lot of good football players here and we're proud of our 53-man roster. Next week we're going to be really excited looking to take on the Vikings."
In the wake of McKinnon's calf strain and Matt Breida's Aug. 9 shoulder separation, the 49ers signed veteran Alfred Morris two weeks ago, reuniting Morris with Shanahan from their Washington days in 2012-13. The only other running back on the current roster is Raheem Mostert, who's more of a special-teams ace.
Shanahahn said Breida returned to practice Saturday, as did tight end George Kittle, who also sustained a shoulder separation in the exhibition opener.
The 49ers signed McKinnon, a former Minnesota Vikings backup, to a whopping four-year, $30 million deal as free agency opened in March. He started the Aug. 9 exhibition against the Dallas Cowboys and was held to minus-4 yards on three carries while also making a 7-yard catch.
