Dallas Cowboys linebacker Sean Lee injured his left knee during practice on Tuesday. The Cowboys are worried that he tore his ACL. He’ll have an MRI on Tuesday to see what the injury is.

Lee was slipping while being blocked by rookie right guard Zack Martin on a screen pass play during practice. After he fell he remained on the ground and was helped off the field by trainers. This isn’t the first time that Lee has been injured. In the last two years he has missed 15 of 32 games and last year he missed five games due to hamstring and neck injuries, according to the Associated Press. In four years in the NFL, he has never played a full season.

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

Last summer Lee signed a six-year extension with the Cowboys for $51 million. In 2011 he led the team in tackles and had 77 tackles in 2012. He led the team with four interceptions last year, according to ESPN.

The Cowboys drafted Anthony Hitchens last month to be Lee’s backup.