ARLINGTON, Texas (AP) - When Sean Lee's left leg slid out from under him and he didn't get up, teammate Barry Church figured the Dallas Cowboys were in for some bad news on the first day of offseason practice.
The fifth-year safety also knew Dallas' defensive leadership was shifting his direction.
''I was right behind him when it happened,'' Church said Wednesday, a day after Lee tore the anterior cruciate ligament in his left knee and put his season in jeopardy. ''And that's one of my best friends on the team. Feels kind of awful. But we have his back no matter what.''
Cowboys coach Jason Garrett refused to offer any details on Lee's injury after participating in a home run derby for charity at the home of the Texas Rangers. The team reported on its website that Lee has been told he has a torn ACL.
Even with a quick recovery from such an injury, Lee wouldn't be ready until the second half of the season. His teammates talked like they were expecting to be without the linebacker who hasn't played a full season in his four years in the league.
''You always have to stay positive,'' cornerback Brandon Carr said of a defense that was plagued by injuries a year ago and finished last in the league and is now without Lee and career franchise sacks leader DeMarcus Ware, who was released in March. ''It's a very unfortunate situation - a guy that works his butt off each and every day. He brings a lot to the table, but at the same time it's the next man up mentality.''
First on that list might be Church, who assumed a leadership role when Lee missed the final three games last year with a neck injury and is versatile enough to fill the role of a linebacker for a few snaps.
Church missed the last 13 games in 2012 with a torn Achilles tendon before bouncing back last year with the first 16-game season of his career. If the Cowboys are to break a three-year rut of 8-8 finishes and a four-year playoff drought, plenty will fall to him.
And Lee has already told him that.
''Me and a couple of the other guys are going to have to take over that defense, just lead the younger guys in the right direction,'' Church said. ''Feel like we can do that.''
The oft-injured Lee went down during the 11-on-11 portion of Tuesday's practice. His left leg slid out from under him as rookie guard Zack Martin was closing in for a block, and Martin rolled over him.
Contact isn't allowed in offseason workouts, but blockers frequently engage linemen and linebackers before plays stop in 11-on-11 drills. Lee was already on his way to the ground when Martin lunged over him.
''I don't really know what happened,'' said Martin, the Cowboys' first-round pick at No. 16. ''Two guys playing hard, running to the ball.''
The 27-year-old Lee hasn't played a full season in four years. The middle linebacker has missed 15 games over the past two years, including five of the final six games in 2013.
Lee signed a six-year extension worth up to $51 million last year, but durability-based incentives are built into the deal. He's already likely to miss out on a bonus in 2015 for not playing enough snaps in 2013 or the coming season.
When healthy, Lee is among the most productive linebackers in the league, leading his position with 11 interceptions since entering the league in 2010 despite missing 18 games.
But his injury history dates to his college days, when he also tore a knee ligament during offseason workouts. The right ACL injury in spring practice in 2008 kept him out that season, pushing his senior year to 2009.
He was already back at the team's practice facility a day after his latest injury, going over plays with some of the younger players. That group includes rookie fourth-round pick Anthony Hitchens, who could get a shot at replacing Lee.
''He wants to win, if he's on the field or not,'' Hitchens said. ''It means a lot to me, being a young guy that he's helping me out already. He's been here for a while, so he knows all the pieces of the puzzle. So I always go to him and ask questions.''
Not that he wants it, but Lee apparently will have plenty of time to answer them.
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