Extent of Sean Lee injury unknown

— -- IRVING, Texas -- Sean Lee injured his left knee during the Cowboys' first organized team activity workout Tuesday, the latest injury for the middle linebacker.

Coach Jason Garrett did not know the severity of the injury. He said Lee would undergo an MRI and further testing with team doctors.

"Sean has had to deal with injuries throughout college and also here in the NFL," 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."

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.

He missed two games as a rookie in 2010 with a hamstring injury and one game in 2011 with a dislocated wrist, playing most of that season wearing a cast. Toe surgery forced him to miss 10 games in 2012.

Lee missed the 2008 season at Penn State because of a torn right ACL suffered in a noncontact drill.

The Cowboys signed Lee to six-year extension last summer worth as much as $51 million. It included a $10 million signing bonus. The base value of the deal is worth $42 million, and Lee could earn up to $9 million more if he plays in at least 80 percent of the snaps.

Lee is vital to the Cowboys' defense. He led the team in tackles in 2011 despite the wrist injury then finished fourth on the team in 2012 with 77 despite playing in only six games. Lee led the Cowboys in interceptions last season with four.

"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.

The Cowboys used four middle linebackers last season because of injury: Lee, Justin Durant, Ernie Sims and DeVonte Holloman. Dallas selected Anthony Hitchens in the fourth round of this month's draft to be Lee's backup. Hitchens played weakside linebacker at Iowa.

News of Lee's latest injury spread quickly Tuesday.

Said Washington Redskins quarterback Robert Griffin III on Twitter: