On the first day of organized team activities, Dallas Cowboys linebacker Sean Lee was helped off the practice field by trainers after suffering a knee injury that will sideline him for the entire 2014 season.

An MRI confirmed what the Cowboys had feared -- that Lee suffered a torn ACL and will not return for the upcoming season, ESPN.com reported.

Video showed Lee expressing his frustration with is another setback in his young career. He apparently hurt his left knee while being blocked by rookie first-round draft pick Zach Martin.

It's the third time Lee has had an ACL injury and the second time it happened to his left knee. Lee tore his right ACL in college and partially tore his left ACL in 2009.

Last year, Lee sat out the final three games and five games overall with neck and hamstring problems. He missed all but six games of the 2012 season with a toe injury and was sidelined for one game in 2011 with a wrist injury. Since Lee signed as the team's second-round draft pick in 2010, he has yet to play a full season.

However, Lee has shown enough ability on the field that he received a six-year extension before last season that could be worth $51 million, but he has to play 80 percent of the team's snaps to reach the deal's escalators, which is notable considering his struggles with injuries.

"Obviously, I've had injuries and I've said I need to find a way to stay on this field," Lee told the Cowboys' official website. "And the contract is extremely fair. If I'm on the field the whole year, I'll be able to get paid. But if I do have injuries, it protects the Cowboys and I think that's fantastic, especially with the injuries I've dealt with in the past.

"Hopefully, I'll find a way to be on the field every single year. That's my goal. So I think it's extremely fair."

Lee had 99 tackles and four interceptions in 11 games for Dallas last season. Lee's absence will leave a big hole in the middle of the Cowboys defense.

While Lee left Tuesday's practice, quarterback Tony Romo was back on the field throwing passes and working on handoffs just six months after undergoing back surgery.

The 34-year-old Romo, who missed the final game of the 2013 season after the surgical repair of a herniated disk, did not participate in team drills later in the session. Backup and Cowboys newcomer Brandon Weeden worked with the first team, according to reports.

Romo is expected to take about a third of the snaps during the OTAs, ESPN reported.