Oklahoma State starting left tackle Devin Davis suffered a season-ending knee injury, a source told ESPN.
The injury occurred more than a week ago, according to a source.
The Cowboys open the season Saturday against Mississippi State in Houston.
The Oklahoman first reported Davis' suffered a torn ACL, but a source told ESPN it was unknown what ligament damage Davis has suffered.
As a redshirt freshman in 2012, Davis played in only seven games, but was seen as a rising star on the line. Even though Oklahoma State returned Parker Graham, who made 11 starts at left tackle last year, the Cowboys inserted Davis into the starting lineup at left tackle and moved Graham to right guard during the offseason
With Davis out, sliding Graham back to left tackle figures to be one of the options the Cowboys will consider as they prepare for their opener with Mississippi State this weekend.
While the Davis injury is a blow, Oklahoma State has been in this position before. The Cowboys lost starting left tackle Michael Bowie before the start of season in 2012 due to a violation of team rules.
ESPN.com's Jake Trotter contributed to this report.