The team did not announce further details or disclose a timeline for either player's return.
The loss hurts the Cowboys, but it's hardly a huge hit.
Thomas was a first-year starter in 2010, finishing fourth on the team with 63 tackles and three interceptions
The Cowboys will need him in Week 2 against Arizona's high-flying pass offense which features one of the nation's best pass-catch combos in Nick Foles and Juron Criner. If Thomas cannot go against Arizona, it's not all bad for Oklahoma State.
Thomas' likely replacement, Daytawion Lowe, has long been one of the team's budding stars, but has been prevented from breaking out because of the experience ahead of him. He could start for a few teams in the league, but now it looks like he'll get a chance with his current team.
Lowe missed all of last season with a knee injury, but he'll get a chance to prove he's officially back with a debut on Saturday night against Louisiana-Lafayette.
Harrison's biggest moment of 2010 came in Justin Blackmon's absence in a game against Kansas State. Harrison caught a jump ball to help the Cowboys beat the Wildcats, 24-14.
This time around, though? Blackmon's on the field and Oklahoma State's deep receiving corps should be fine without his 14 catches for 135 yards and three scores.
His absence hurts, but for now, the Ragin' Cajuns don't have enough to pose a serious threat.
If Thomas and Harrison are ineligible into next week or longer? The impact will grow.