Minnesota Vikings running back Dalvin Cook has torn the ACL in his knee and will miss the rest of the season.
Cook, who was a second-round pick in the 2017 draft, was injured early in the third quarter of Minnesota's 14-7 loss to the Lions, hurt his knee while cutting to avoid Detroit safety Tavon Wilson.
As he was tackled by Wilson, the ball was knocked loose from Cook's arm, and Detroit recovered the fumble.
Cook, who had rushed for 66 yards on 13 carries prior to the injury, was helped from the field and did not return.
Afterwards, Viking head coach Mike Zimmer said: "I expect him to come back and be the same that he was,"
NFL Network are reporting the initial diagnosis by Vikings medical staff is that Cook has suffered almost a complete tear of his ACL.
Losing Cook would be a huge blow to a Vikings offence that has already been without starting quarterback Sam Bradford for three weeks, though he could return in Week Five against the Chicago Bears on Monday Night Football.
Cook is the NFL's third leading rusher with 354 yards on 74 carries (4.8 YPC) with two touchdowns, while he has also caught 11 passes for 90 yards.
Latavius Murray assumed the majority of the workload in Cook's absence on Sunday, rushing for 21 yards on seven attempts.
"I don't think it really matters how it [happens that I'm starting]. I've got to be ready," Murray said. "Just like today, unfortunate things happen, so it's not about whether or not what rhythm I have. I need to be ready when my number is called."
He added: "Any time a teammate goes down, especially in this group and a friend of mine, you feel bad for them."