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FRISCO, Texas -- Dallas Cowboys rookie linebacker DeMarvion Overshown suffered a torn anterior cruciate ligament in his left knee in the first quarter of Saturday's preseason loss to the Seattle Seahawks, according to a source.

Overshown was hurt while making a tackle in the second series of the game when the knee appeared to buckle. He was able to walk off the field on his own but was taken to the locker room via a cart and did not return to the game.

An MRI on Sunday confirmed the team's fears on Saturday night.

"He's had an incredible camp," coach Mike McCarthy said after the 22-14 loss. "Seems like we talked about him every other day. I hope he's OK."

Overshown was the Cowboys' third-round draft pick, No. 90 overall, in the spring after a five-year career at Texas in which he played safety, linebacker and pass-rusher. The Cowboys planned to use that sort of versatility heavily on special teams by using Overshown as a core player, but he also showed in training camp practices that he could potentially help on defense.

His injury leaves the Cowboys even thinner and more inexperienced at linebacker with Leighton Vander Esch as the most experienced player as he is entering his sixth season. Micah Parsons is still listed as a linebacker, but most of his action comes as an edge player. Malik Jefferson suffered a foot injury in the preseason opener.

The Cowboys are counting on Damone Clark to start next to Vander Esch but now will hope Jabril Cox and/or Devin Harper can play bigger roles or they'll be forced to look for veteran free agent help.

Rookie tight end John Stephens Jr., who was pushing for a roster spot, also suffered a torn ACL in the Seattle loss.