Sankey sustained a torn ACL during Thursday's preseason win at Buffalo, Ian Rapoport of NFL Network reports.
The Vikings held out Latavius Murray and Jerick McKinnon on Thursday, providing Sankey with a window in which to impress the coaching staff. Sankey received four carries for 14 yards and gathered in four of five targets for 15 yards before suffering a knee injury, which has been revealed to be season-ending. Injured reserve is almost certainly in the 2014 second-round pick's future.