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Ryan's departure comes after the Cowboys finished 8-8 and out of the playoffs. Dallas finished the season 19th in the NFL in yards per game allowed, but was 24th in the league in points per game allowed. The Cowboys had a chance to make the playoffs, but lost their final two regular season games due in large part to the defense, which allowed 31 points per game over those two contests.
While some might speculate that Ryan could join his brother Rex in New York, it almost certainly won't be as defensive coordinator. The Jets recently promoted defensive backs coach Dennis Thurman to defensive coordinator and that isn't likely to change.