Lee has already missed 18 games in the first four years of his NFL career. He is now on his way to missing the entire 2014 season.
While at Penn State, Lee tore his right ACL in April of 2008. He then torn his left ACL during the 2009 season.
The Cowboys have not released any information.
“Sean has had to deal with injuries throughout college and also here in the NFL,” Cowboys coach Jason Garrett said following Lee’s injury on 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 Cowboys gave Lee a six-year extension last summer worth $51 million.