The Cowboys have made no official statement regarding the results of the MRI performed on Lee's injured left knee, but the linebacker has been told the exam confirmed he tore the anterior cruciate ligament in the knee and he is not expected to play in the upcoming 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," coach 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."
The NFL Players Association has contacted Cowboys players to gain a sense of what happened in the non-contact workout, as is standard procedure in these types of incidents. Multiple sources told ESPN's Ed Werder that the prevailing opinion within the team is that Lee suffered the injury when he slipped, which occurred just before rookie first-round draft pick Zack Martin collided with the linebacker on a screen pass, sending both to the ground.
The union has not requested videotape of the practice. By terms of the CBA, that can be done only if it is alleging a team violated a rule. So the mere request is an allegation. The NFLPA is not to that point yet with this situation. Teams determined by the NFL to have committed violations have been punished by losing practice sessions in the past.
After the play, 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," 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 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.
The Cowboys signed Lee to a 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 $42 million, and Lee could have earned up to $9 million more if he played 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.
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 year's draft to be Lee's backup. Hitchens played weakside linebacker at Iowa.
News of Lee's latest injury spread quickly Tuesday.
Put away the rivalry for a minute Prayers sent up for Sean Lee
- Robert Griffin III (@RGIII) May 27, 2014
ESPN NFL Insiders Adam Schefter and Ed Werder contributed to this report.