Jets head coach Todd Bowles told reporters of the diagnosis Monday after a second opinion. McCown will undergo surgery on the injury.
The quarterback also appeared to hurt his abdominal section earlier in the day before returning. He finished just 6-of-12 for 46 yards, plus an interception and a fumble.
Bryce Petty replaced McCown under center, facing what was already a 20-0 deficit at the time.
McCown, 38, played just one game in 2016 for the Cleveland Browns before breaking his collarbone in Week 2, leaving Cody Kessler as the starter. The Browns abstained from putting him on injured reserve, and McCown was able to return in Week 8, though he played in just five games overall, completing 54.5 percent of his passes while throwing for 1,100 yards, six touchdowns and six interceptions.
The Jets signed him in the offseason, becoming the eighth team McCown has played for in his NFL career. He came into the season as the presumed starter and was expected to be the bridge starter until either Petty or Christian Hackenberg was ready to take the reins.
McCown has been surprisingly solid, coming into the game throwing for 2,880 yards, 18 touchdowns and eight interceptions, helping lead the Jets to a better record than many expected prior to the season.
Ready or not, Petty will now be called into action.
While the Jets surprised many by being competitive this season, losing McCown does not bode well for their chances of success down the stretch.