The Dallas Cowboys fear star linebacker Sean Lee tore his left ACL during the team's first organized team activity workout Tuesday, a source familiar with the injury told ESPN.

Lee will undergo an MRI on Tuesday to see if the Cowboys' fears are a reality, the source said.

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," 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."

On the play he was injured, Lee appeared to be slipping while being blocked by rookie right guard Zack Martin on a screen pass. Lee remained on the ground and needed to be helped to the locker room by members of the training staff.

"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.