New York Giants cornerback Terrell Thomas suffered an anterior cruciate ligament injury to his right knee during practice on Sunday.
An MRI and examination by team physician Dr. Russell Warren on Monday confirmed the injury.
"He came in this morning with a swollen knee, went down to HSS (Hospital for Special Surgery)," Giants head coach Tom Coughlin said. "He walked up to me after practice, he thought he felt good. He didn't really seem to have any issues and then this morning, when he came back and he was swollen."
Thomas, who signed a four-year $28.4 million deal in March, spent the entire 2011 season on injured reserve.
The 27-year-old led the Giants in tackles and interceptions in both 2009 and 2010, but tore his ACL in a preseason win over the Bears last August, causing him to miss all of the Super Bowl XLVI-winning campaign.
A second-round pick in 2008, the USC product has recorded 231 tackles, 11 interceptions and 38 passes defended in 44 career games -- all with New York.
Thomas also tore the ACL in his right knee in 2005 while at USC.