Clint Session is out for the season with what was described as a head injury, which usually means a concussion.
He was hurt Sunday in Cleveland.
Last week, the Jaguars lost cornerback Rashean Mathis for the year with a torn ACL. They also put offensive tackle Eben Britton on injured reserve, but he hadn't played in a month due to a back ailment.
Session will be replaced on the roster by veteran linebacker Kevin Bentley, who is in his 10th NFL season and has spent times with the Cleveland, Seattle, Houston and San Diego.
He played the last three seasons with Houston and was in San Diego in training camp this year before he was released on the final cuts.
Session was signed as a free agent after the lockout, and he started five of nine games and missed one game with an elbow injury. He didn't start when the Jaguars opened with a nickel defense.
Session's lineup spot will be filled by Russell Allen, who made his only start of the season at Pittsburgh when Session was out with an elbow injury.
The Jaguars are thin at linebacker because backup Kyle Bosworth went on IR last week and they have only five linebackers on the team.
Jacksonville now has 18 players on injured reserve.