Jerick McKinnon has encountered a setback in his return from a torn ACL.
His status for the start of the season with the San Francisco 49ers is up in the air.
McKinnon returned to practice Tuesday afternoon but the session apparently didn't go that well for him and his surgically repaired right knee.
"Yesterday we brought him out and it's a real bummer because he keeps getting to the final step," John Lynch said. "And the final step is actually playing NFL football, particularly at his position where you have got to make hard cuts. You've got to put your foot in the ground. And we did it a month ago. And we got to that step and he kind of regressed and I would say yesterday we had a similar situation. So we're trying to get to 'OK, what's really the root cause of the problems that he's having?'
"Yesterday was not encouraging from that standpoint for Jet. What that means, we're not sure yet. We're working hard to find that out."