Aaron Ross has torn Achilles tendon
Ross, 31, who had played six of his seven NFL seasons for the New York Giants, was expected to compete for the Ravens' third cornerback spot. He was signed last month after trying out at minicamp.
Last season, Ross was placed on injured reserve after four games because of a back injury.
"That's a tough break for him," Ravens coach John Harbaugh said.
Harbaugh said there is no urgency to sign another free-agent cornerback. The Ravens' top backups include Chykie Brown, Asa Jackson and Dominique Franks, who was placed on the non-football injury list after failing the conditioning test.
"We have good players at corner. The guys are competing," Harbaugh said. "I expect a third corner from one of the guys who are already here. But if there is somebody out there who can bolster the competition, we'll bring them in."