NEW YORK -- Noah Syndergaard did not make his second scheduled Minor League rehab start on Sunday for an undisclosed health reason.

Manager Luis Rojas initially made the announcement on Sunday morning, chalking Syndergaard’s absence up to a “non-baseball-related injury.” Multiple sources later clarified that Syndergaard is not dealing with an injury, but instead an illness. Rojas confirmed that Syndergaard is vaccinated against COVID-19.

“He’s frustrated,” Rojas said.

No other Mets players or employees needed to be isolated in response to Syndergaard’s infection, according to a team official. Syndergaard had been working out regularly with Mets staffers and traveling with the team while on the injured list, recovering from Tommy John surgery.

That recovery advanced significantly last week, when Syndergaard began his second Minor League rehab assignment with High-A Brooklyn after elbow soreness forced him to cut short his first assignment back in May. Syndergaard threw one inning Thursday night for the Cyclones, with plans to go an inning-plus on Sunday.

Now, Syndergaard’s timeline has been backed up indefinitely. The Mets had hoped to have Syndergaard back from the IL in early September in a relief role, which would not require a significant buildup in the Minor Leagues. Whether this setback costs Syndergaard days or weeks off that timeline remains to be seen.

“We haven’t committed to anything yet,” Rojas said. “We’re just waiting for clarification of when his next outing will be, and I think that will give us an idea of what the next step is.”

Syndergaard, who celebrated his 29th birthday on Sunday, has not pitched in a Major League game since September 2019. He underwent Tommy John surgery the following March and has been recovering ever since. In five seasons for the Mets, Syndergaard is 47-30 with a 3.31 ERA. He can become a free agent for the first time after this season, putting his longer-term future in doubt.

“I can’t imagine leaving New York or leaving the Mets,” Syndergaard said last week, in his first extended public comments since surgery. “I love the culture that New York has to offer, the grit and tenacity of all the citizens and fans. Especially ever since 2015 -- that special run will forever be engrained in me. It was the best time in my life, and I really want to get back to that.”