Ravens starting cornerback Lardarius Webb left the game in the first quarter and headed to the locker room for further examination.
Webb immediately grabbed his left knee and was on the ground in obvious pain. Trainers helped him off the field, and he barely put any weight on his leg. The Ravens said he's out for the game.
According to a league source, the fear is that Webb has a torn left anterior cruciate ligament. A magnetic resonance imaging exam will be conducted Monday to confirm the initial diagnosis.
The Ravens inserted Corey Graham as the nickel with Jimmy Smith taking Webb's place opposite Cary Williams.
Meanwhile, Pro Bowl defensive tackle Haloti Ngata left the game in the second quarter with a sprained knee. He limped off the field and headed to the locker room for further examination from team doctor Leigh Ann Curl. He later returned and has recorded six tackles and a sack despite playing in obvious discomfort.
Cowboys rookie cornerback Morris Claiborne, a first-round draft pick, injured his left knee during Ravens wide receiver Torrey Smith's touchdown catch toward the end of the second quarter. Claiborne was helped off the field by trainers, walking very gingerly as far as applying weight to his left leg.
The Ravens are already playing without outside linebacker Terrell Suggs, who's on the physically unable to perform list with a partially torn right Achilles tendon.
Webb tore the anterior cruciate ligament in his right knee as a rookie and had it surgically repaired by Dr. James Andrews.
Webb was signed to a $50 million contract extension last spring and is indisputably the Ravens' top cornerback.
NOTE: Middle linebacker Ray Lewis left the game late in the fourth quarter and didn't return. He was walking around the bench area, but didn't have his helmet on. There was no immediate diagnosis on Lewis' status, but he appears done for the day.