According to ESPN's Rich Cimini, a source said Mosley's decision is due to "family health concerns."
Mosley signed a five-year, $85 million contract with the Jets in free agency last offseason. His first season in New York after five years with the Baltimore Ravens was a bust, as a groin injury limited him to just two games.
Mosley is perhaps the highest-profile player to opt out of the 2020 season thus far. Other notable players who have opted out include New York Giants offensive tackle Nate Solder, Kansas City Chiefs running back Damien Williams, Buffalo Bills defensive tackle Star Lotulelei, Chicago Bears defensive tackle Eddie Goldman and seven members of the New England Patriots, including linebacker Dont'a Hightower, safety Patrick Chung and offensive tackle Marcus Cannon.
With the Jets trading All-Pro safety Jamal Adams to the Seattle Seahawks in late July, Mosley was set to enter 2020 as the team's clear top player and leader on defense.
While Mosley has yet to accomplish much in New York, he was a four-time Pro Bowler in his five seasons with the Ravens. During his tenure in Baltimore, he averaged nearly 116 tackles, two sacks and two interceptions per season.
Now that Mosley is out of the fold for 2020, the Jets will be tasked with finding two starters at inside linebacker among Blake Cashman, Avery Williamson, James Burgess, Neville Hewitt and Patrick Onwuasor.
Burgess and Hewitt were the most productive among them for the Jets last season, as Burgess registered 80 tackles in 10 starts and Hewitt made 75 tackles in 10 starts. Cashman also showed some flashes as a rookie fifth-round pick with 40 tackles in seven games, including five starts.
The Jets haven't made the playoffs since 2010 and would have been hard pressed to compete in what should be a strong AFC East this season with the Bills and Miami Dolphins both improving and the Patriots remaining a constant threat.
With the Jets losing their top two defensive players in the matter of a week, 2020 could be a major struggle for Gang Green.