There were a lot of crossed fingers in Minnesota yesterday, but the team’s worst fears regarding rookie running back Dalvin Cook have been realized.

The Minnesota Vikings lost to the Detroit Lions, 14-7, at home on Sunday, but that defeat is not as crushing for the team and its fans as the moment when Cook went down with a serious-looking injury.

Cook sustained a knee injury in the second half of the contest. According to Ian Rapoport and Tom Pelissero of the NFL Network, initial tests on Cook suggested an ACL tear for the running back, but today the MRI confirmed it, per ESPN’s Courtney Cronin. Head coach Mike Zimmer said he’ll undergo surgery to repair the left knee.

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Cook had 66 rushing yards and a touchdown on 13 carries against the Lions prior to leaving the game. His injury is another wrench thrown into the window of the Vikings’ offense that is already having a difficult time dealing with the absence of injured quarterback Sam Bradford.

A prolonged absence for Cook means that Latavius Murray will likely inherit much of the work in the backfield. Murray signed a three-year, $15-million contract with the Vikings back in March.