The team did not specify if Demps has been placed on season-ending injured reserve or the new shortened injured reserve, where a player could return later in the season.
Demps was signed on Aug. 17. In the fourth quarter of Wednesday's preseason finale against the New York Giants, he came up limping after a tackle and had his knee/ankle checked out on the sideline.
He didn't return to the game, but said in the locker room that he was OK.
Demps, the Olympic silver medalist, offered a potentially explosive element to the Patriots' return game. He also projected to add depth at running back.