"There's just too many uncertainties, too many unknowns right now to go out there," Stroman said, per Tim Britton of The Athletic.
The 29-year-old had been out to start the season with a calf injury but insisted he was back to 100 percent before making his decision to opt out.
Tim Healey of Newsday noted the issues that caused him to make his decision:
"Strictly worries about COVID" caused him to opt out, not any lingering effects of his injury, per Justin Toscano of the Record.
Stroman was heading into his first full season with the Mets after coming over in a trade with the Toronto Blue Jays ahead of last year's deadline.
Just before the start of the regular season, the right-hander was sidelined by a tear in his calf. He was placed on the injured list and threw multiple simulated games while working his way back to the field.
He was scheduled to throw another simulated game on Tuesday, per Healey.
Despite not making any appearances in 2020, he accrued enough service time to hit free agency this offseason, per Jeff Passan of ESPN.
If he signs elsewhere, Stroman will have made just 11 starts for New York, producing a 3.77 ERA in 59.2 innings. It will be a disappointing return after giving up two pitching prospects in the trade last July: Anthony Kay and Simeon Woods-Richardson.
The right-hander is also the second Mets player to opt out after the start of the season, joining outfielder Yoenis Cespedes.
Stroman has had an up-and-down career since reaching the majors in 2014. He earned Cy Young votes in 2017 and was an All-Star last season thanks to his hot start with Toronto, tallying a 2.96 ERA in 21 starts.
Injuries and inconsistency also have led to some lost seasons over his career.
There should still be an active market for his services in free agency during the upcoming offseason.