The Cowboys acquired Cassel in a trade from the Buffalo Bills after Tony Romo suffered a broken left collarbone in a Week 2 win against the Philadelphia Eagles. Cassel was inactive for his first two games and served as Brandon Weeden's backup before taking over as the starter Oct. 25 against the New York Giants.
The Cowboys won only one of Cassel's seven starts. In four starts he failed to throw for more than 200 yards and was yanked in favor of Kellen Moore on Dec. 19 against the New York Jets. Moore started the final two games of the season with Cassel as the backup.
The 33-year-old Cassel finished the season having completed 119 of 204 passes for 1,276 yards, with five touchdown passes and seven interceptions.
ESPN Cowboys reporter Todd Archer contributed to this report.