Dallas Cowboys linebacker Sean Lee is feared to have suffered a torn ACL in his left knee during the team's first organized team activity on Tuesday, a team insider informed ESPN.

Lee, who is considered one of the key cogs of the Cowboys defense, is scheduled to undergo MRI examination to see the extent of the damage in his left knee. The 27-year old pass-rusher already tore his right ACL in 2008 and partially suffered a torn ACL in his left knee in 2009.

"Sean has had to deal with injuries throughout college and also here in the NFL," Cowboys head coach Jason Garrett said earlier Tuesday on ESPN. "He really does everything he can to get himself ready to play. Unfortunately, he had to deal with a few of those things, but he's as mentally tough as an individual I've ever been around. Whatever the circumstances are, he always puts his best foot forward and strives to be his best. I don't anticipate this being any different."

According to sources, Lee suffered the injury after he slept while being blocked by rookie right guard Zach Martin on a screen-pass play. Lee then grimaced in pain as members of the training staff escorted Lee towards the locker room.

"I don't really know what happened," Martin said. "Two guys playing hard, running to the ball. We'll see what happens. ... Hope for the best. He's obviously a great player, leader of the defense.

Lee, who recorded 99 tackles and four interceptions last season, could miss the entire season, a news that would surely be a big blow for the Cowboys' defense.

"You don't want to see any man go down like that, but we'll wait and see and see how he is," defensive coordinator Rod Marinelli said in the interview on Tuesday.