Texans star defensive end J.J. Watt injured his left knee during the first quarter Sunday night against the Kansas City Chiefs.
Watt was down on the field for a while in obvious pain.
The three-time NFL Defensive Player of the Year was helped off the field by trainers and was unable to put weight on his leg.
The injury appears serious, but there was no immediate diagnosis.
Watt made a healthy return from a pair of back surgeries that forced him onto injured reserve last season.
Texans outside linebacker Whitney Mercilus has been ruled out of the game with a chest injury.