WASHINGTON — Terry Collins engaged in what may have been, in retrospect, a bit of wishful thinking. Hours before the Mets’ excruciating 11-4 loss to the Nationals, the manager quickly brushed aside concerns about Noah Syndergaard’s balky elbow.

“I think he’s in a good space now,” Collins said, before watching one of the best young pitchers in baseball endure his worst outing of the season.

Syndergaard pulled himself from his last start because of elbow tightness, then took the mound against the division-leading Nationals with hardly a sign of his trademark command. Raw velocity was not nearly enough.

He battled through trouble, then allowed five runs in the third inning. He did not return for a fourth.

In the process, Syndergaard blew an early 4-0 lead. He walked three, a season high, and allowed the Nationals to run wild. They swiped five bases with Syndergaard on the hill — the Nationals added one more later — picking away at what has been one of the fireballing righty’s few vulnerabilities.

From the beginning, Syndergaard appeared destined for a slog, loading the bases with nobody out in the first inning only to escape without damage. But through it all, he looked out of sync with batterymate Travis d’Arnaud.

Twice in critical spots, the catcher jogged to the mound for impromptu meetings, a sign that he and the pitcher struggled to connect.

Syndergaard (8-3, 2.49 ERA) looked rattled. He has allowed 28 stolen bases this season; the next closest pitcher in all of baseball has allowed 14.

By the end of the third, the Nationals had sent nine men to the plate in the inning and scored five runs, chasing Syndergaard after only 71 pitches.

Anthony Rendon’s two-run single off Syndergaard tied it and Danny Espinosa, who singled home the run that put the Nationals ahead for good.

Entering in relief, Sean Gilmartin surrendered five more runs in two innings, ending any hopes of a comeback. The Nationals finished with 17 hits.

It is not how Collins envisioned the Mets beginning a pivotal stretch leading up to the All-Star break. Beginning on Monday night, 11 of the Mets’ next 14 games come against first-place teams. During that span, Collins has emphasized the importance of not ceding any more ground.

Yet, they did just that against the Nationals, who pulled four games clear of the Mets. And to make matters worse, the day only heightened fears about the franchise’s most precious assets.

Pitching has allowed the Mets to sustain themselves despite a spate of injuries that has diminished the lineup. But their biggest strength has shown cracks of weakness.

Matt Harvey and Jacob deGrom haven’t fully been themselves. And before the game, the Mets revealed that Steven Matz has been pitching through a bone spur in his elbow that may require surgery.

Later, Syndergaard capped the misery, adding even more reason to wonder about his elbow.

Notes & quotes: The Mets held a private workout at their complex in Port St. Lucie for former Cuban star Yulieski Gourriel on Monday, assistant general manager John Ricco said. It was attended by GM Sandy Alderson and a group of the team’s top scouts, including special assistant to the GM J.P. Ricciardi. Gourriel, a third baseman, is expected to receive heavy interest for his services . . . Juan Lagares (sprained left thumb) begins his rehab assignment on Tuesday with Double-A Binghamton. The Mets are hoping that Lagares can play through his sprain and avoid surgery . . . Zack Wheeler played catch on Monday for the first time since his rehab from Tommy John surgery was slowed by nerve irritation in his right elbow . . . The Nationals’ Lucas Giolito, one of the top pitching prospects in baseball, will make his big-league debut against the Mets on Tuesday.