Sean Lee is attended to by the athletic training staff after hurting his knee Tuesday. (Tony Gutierrez / AP)

One practice into the new NFL season and the Dallas Cowboys are preparing to be without the man Coach Jason Garrett has called the “heart and soul” of the defense.

Sean Lee, according to the Dallas Morning News and ESPN, has been told that he has a torn left anterior cruciate ligament and he is expected to miss the entire season. The linebacker was hurt on the first day of noncontact organized team activities when he appeared to slip and his leg gave out as he pursued running back DeMarco Murray on a screen pass. Rookie offensive lineman Zack Martin then fell over the top of him. As he was being helped off the field by two trainers, Lee cursed, and seemed to know immediately what had happened. He has an extensive history of injuries, including knee injuries, and he has had season-ending injuries the last two years.

“He’s a special guy, and he’s had to deal with some adversity from an injury standpoint in his career before and he’s come back,” Garrett said immediately after the OTA and before Lee underwent an MRI, “and we’ll evaluate what happens and he’ll respond accordingly.”

The loss of Lee is a terrible blow to the Cowboys, who had the league’s worst defense last year and allowed DeMarcus Ware to depart. These are noncontact drills, but injuries still occur in them. “We understand that we have helmets and jerseys on, and we really emphasize players staying off the ground and how you compete against each other,” Garrett said. “Injuries happen when guys hit the ground. We have to understand who we’re going against, how we’re going against them and making sure guys stay up.”

The Cowboys are expected to make an announcement on Lee today.

H/T Calvin Watkins of ESPN Dallas for the video