Sean Lee has been told that he has a torn ACL in his left knee and is expected to miss the entire Dallas Cowboys' season.

Lee tore his right ACL in April 2008 while at Penn State, then partially tore his left ACL during the 2009 season.

"Sean has had to deal with injuries throughout college and also here in the NFL," Jason Garrett said earlier Tuesday. "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."

Lee appeared to be slipping while being blocked by Zack Martin on a screen pass.

"I don't really know what happened," said Martin, the Cowboys' first-round pick. "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 missed five games last season with hamstring and neck injuries. He was cleared from the neck injury at the start of the offseason program in April.