Matt Harvey sat down in front of his locker on Wednesday afternoon and quietly pulled out a hard-shelled suitcase. He finished the last of his packing, then zipped up his bag, his destination revealed only a few hours later.

Harvey, the embattled Mets righthander, has been placed on the 15-day disabled list with what the team is calling right shoulder “discomfort.” On Thursday, the pitcher will visit a St. Louis-based vascular specialist, Dr. Robert Thompson.

A source told Newsday’s David Lennon that the Mets suspect Harvey may have thoracic outlet syndrome, a relatively rare condition that could mean a variety of outcomes, including season-ending surgery.

Thompson specializes in treating thoracic outlet syndrome (TOS), a general term for symptoms that stem from the compression of blood vessels and nerves.

The Mets released the news a few hours after a 4-2 victory over the Marlins. Club officials will not comment until Thursday.

Harvey’s DL stint is the latest in a series of setbacks in what has been a cursed season, beginning with a bladder infection that he contracted near the end of spring training.

In his year full season coming off Tommy John surgery, the 27-year-old Harvey is 4-10 with a 4.86 ERA. Entering the season as ace of baseball’s youngest and most dynamic staff, he has pitched with little of the poise and confidence that had once defined him.

Through all of his uncharacteristic struggles, Harvey and the Mets firmly maintained that the righthander’s issues have been largely mental and mechanical, the reasons behind what had been diminished velocity earlier in the season.

The Mets identified a hitch in Harvey’s delivery — more reason to believe that the pitcher’s issues did not stem from any health concerns. In his last outing on Monday, Harvey was chased after giving up six runs in 3 2⁄3 innings against the Marlins.

Now, Harvey and the Mets face the prospect of even more uncertainty.

The Cardinals’ Jaime Garcia, former Met Chris Young and one-time Red Sox ace Josh Beckett are among those who have bounced back from surgery to treat thoracic outlet syndrome. All faced a lengthy rehab process.

Twins pitcher Phil Hughes underwent surgery to deal with TOS recently, though he had concerns about having the condition as early as 2011, when he was still pitching for the Yankees.

For some such as former Tigers righty Jeremy Bonderman, thoracic outlet syndrome proved to be career ending.

The Mets hope to learn if Harvey indeed is diagnosed with the condition. If he is, surgery could be a season-ending proposition.

Thompson is the same doctor that diagnosed former Met Dillon Gee with an artery ailment in 2012 and performed surgery that ended the righty’s season.

Seth Lugo, the 34th-round draft pick who made his big league debut just this week, has been recalled from Triple-A Las Vegas. Harvey was slated to start on Saturday in a critical four-game series against the Nationals, though it’s unclear who will ultimately take his spot.

The Mets staked their World Series aspirations on a pitching staff, taking great pains to protect their collection of precious gems. Nevertheless, the group has shown vulnerability.

Harvey is on the disabled list and Jacob deGrom has had to endure diminished velocity. All-Star Noah Syndergaard is pitching through a small bone spur in his right elbow that causes intermittent pain.

And until Harvey’s condition was revealed, lefthander Steven Matz appeared to be the standing on the shakiest ground.

Like Syndergaard, Matz is pitching through a bone spur in his elbow. But his is larger and will require surgery. The Mets are hoping to delay it until the offseason, though it’s possible that the pain proves too overwhelming for Matz to press on.

Righty Zack Wheeler had been expected to complete his recovery from Tommy John surgery after the All-Star break. But setbacks — the latest being a nerve issue in his elbow — pushed him off schedule.

Wheeler only recently resumed throwing. He is long tossing but has yet to progress to bullpen sessions, leaving him more than a month away from being ready for a return.