He is expected to undergo surgery, at which time the Cowboys will know whether there is damage to his anterior cruciate ligament. Dez Bryant suffered a tibial plateau fracture last year and missed three games.
Hitchens suffered the injury with 19 seconds left in the first half of Saturday's preseason game against the Oakland Raiders, and the initial fear was a torn ACL. He underwent additional testing Sunday.
It is possible Hitchens could start the year on injured reserve. Teams are allowed to recall two players from IR during the season.
The Cowboys will count on veteran Justin Durant, Jaylon Smith and potentially Damien Wilson in Hitchens' absence. Hitchens started every game last year and was credited by the coaching staff with 104 tackles, 1.5 sacks, four tackles for loss, five quarterback pressures and two pass breakups.