It's not clear how Greer was hurt, but it happened during the Browns' 12-play, 96-yard drive that was capped by Joshua Cribbs' 2-yard touchdown run that put Cleveland ahead 13-0. Greer made a tackle during the drive, stopping Braylon Edwards on an 18-yard catch.
The Bills did not announce the severity of the injury, except to say that Greer would not return. He'll have further tests Tuesday.
Greer has been Buffalo's most dependable cornerback this season, with 42 tackles, a team-leading nine passes defensed and two interceptions, which he's both returned for touchdowns.
The Browns lost two defensive starters to injuries: end Shaun Smith (calf) and safety Sean Jones (ankle). Smith did not return after aggravating a calf injury while tackling Marshawn Lynch on the Bills' first possession of the third quarter. He was limping as he left the field and went immediately to the Browns' locker room. Smith was initially hurt in the first half.
Jones did not return after being hurt in the first half.
Information from The Associated Press contributed to this report.