The Dallas Cowboys have lost their best defensive player to a serious injury again.

NFL Media Insider Ian Rapoport reported Tuesday night that linebacker Sean Lee tore his ACL at the team's OTA session and is expected to miss the 2014 season, according to a source who has seen the MRI results. Lee's left knee buckled when he was run over by first-round pick Zack Martin during a drill that was supposed to be non-contact.

It's a devastating blow for Lee, who has missed 15 games over the last two seasons due to wrist, neck and hamstring problems. When he's fully healthy, Lee mixes great run pursuit with rangy pass coverage as well as any linebacker in the league. He was expected to anchor a Cowboys defense that looks particularly soft up the middle with holes at defensive tackle and questions next to Lee at linebacker.

The Cowboys were set to draft Ryan Shazier in the first round before the Pittsburgh Steelers took the linebacker off the board. Dallas was hoping to add a playmaker next to Lee after Bruce Carter disappointed the coaching staff one season ago.