Dallas Cowboys linebacker Sean Lee could miss the 2014 season after tearing a ligament in his left knee in the first off-season practice.
The team hasn't announced results of an MRI but reported on its website Wednesday that Lee has been told he has a torn anterior cruciate ligament.
The oft-injured Lee went down during the 11-on-11 portion of Tuesday's practice. His left leg slid out from under him as rookie guard Zack Martin was closing in for a block, and Martin rolled over him.
With Lee's injury and the release of franchise sacks leader DeMarcus Ware, the Cowboys could be without their top two defenders after finishing last in the league in total defense a year ago.
Contact isn't allowed in off-season workouts, but blockers frequently engage linemen and linebackers before plays stop in 11-on-11 drills. Lee was already on his way to the ground when Martin lunged over him.
"I don't really know what happened," said Martin, the Cowboys' first-round pick at No. 16. "Two guys playing hard, running to the ball."
The 27-year-old Lee hasn't played a full season in four years. The middle linebacker has missed 15 games over the past two years, including five of the final six games in 2013 with hamstring and neck injuries.
Lee signed a six-year extension worth up to $51 US million last year, but durability-based incentives are built into the deal. He's already likely to miss out on a bonus in 2015 for not playing enough snaps in 2013 or the coming season.
When healthy, Lee is among the most productive linebackers in the league, leading his position with 11 interceptions since entering the league in 2010 despite missing 18 games. But his injury history dates to his college days, when he also tore a knee ligament during off-season workouts. The right knee injury in spring practice in 2008 kept him out that season, pushing his senior year to 2009.
The Cowboys had a first-round grade on him for the 2010 draft but got him late in the second round because of the injuries.