He later explained his biggest concern was the risk of spreading the coronavirus to his family.
"Because of COVID, my family is my support system with my son," Mosley said. "So, I didn't want to risk that obviously. But I think just looking at the big picture, for me, I want to make sure I'm mentally and physically the best person I can be so I can be that leader I know I can be."
The linebacker is one of two Jets players to opt out as of Tuesday, with offensive lineman Leo Koloamatangi also sitting out the year.
It will especially be difficult to replace Mosley, who is a difference-maker when on the field. The 28-year-old earned four Pro Bowl selections in five years with the Baltimore Ravens and was named second-team All-Pro four different times.
He topped 100 tackles in all but one year in that stretch.
This production earned him a five-year, $85 million deal with the Jets, although his first season with the new team was limited to just two games. After returning an interception for a touchdown in Week 1, he suffered a groin injury that kept him out most of the year.
New York will once again have to play without him for the entire 2020 season, likely relying on James Burgess, Neville Hewitt and Patrick Onwuasor to anchor the defense. Meanwhile, Mosley will get the chance to focus on returning to full strength in 2021.
"I'm out all year so I have all this time to mentally get better, physically get better, kind of re-adjust and kind of find that flame again," he said.