But Tuesday night at Citi Field, Jake Irvin’s brief lapse of focus snowballed into a painful second inning during a 7-5 loss to the New York Mets that opened a three-game series.

Irvin recorded the first two outs of the second, then walked Jeff McNeil — the Mets’ No. 8 hitter — on five pitches. It was no big deal until Harrison Bader and Francisco Lindor singled to give the Mets a 1-0 lead. Two pitches later, Brandon Nimmo hit a three-run homer to the opposite field. A harmless two-out walk had turned into a four-run inning.

“It’s my job to keep guys off the base paths,” Irvin said. “And walking guys with two outs is unacceptable.”

On the homer, rookie left fielder James Wood took a few steps back and appeared ready to settle under a flyball. But he kept moving back as the ball kept carrying and eventually cleared the fence. Per Baseball Savant, it would have been a home run in just 14 of 30 MLB parks.

Irvin lasted six innings and allowed six runs — all of them with two outs. (The Mets added a run against Jordan Weems in the eighth.) The 27-year-old was an all-star candidate who has impressed throughout his second MLB season, but Tuesday’s rough outing boosted his ERA to 3.13 as he equaled the most runs he has allowed this season. His other six-run outing came April 24 in a loss to the Los Angeles Dodgers, who were his opponent for a second consecutive start.

Irvin faced the same scenario Tuesday. On Thursday at Nationals Park, he pitched eight one-hit innings and struck out eight in a 1-0 win over the Mets. Jose Quintana opposed him that day and tossed seven innings of scoreless ball himself, allowing four hits.

Quintana was even better Tuesday, allowing one hit in seven innings while striking out five. He retired the side in order in six innings. The only time he didn’t was the third, when Luis García Jr. opened the inning with a single to center and advanced to second on an error by Bader. A hit-by-pitch and a walk loaded the bases with two outs for Wood, who grounded out on the first pitch.

“He wasn’t missing in the middle,” catcher Keibert Ruiz said of Quintana. “He was just attacking the bottom of the zone. His breaking ball was good when he got behind in the count.”

When Quintana exited, the Nationals (42-50), who have lost three in a row, found some life. A García double greeted reliever Adam Ottavino in the eighth, then pinch hitter Ildemaro Vargas slugged his first home run of the season. In the ninth against Reed Garrett, Juan Yepez doubled before Ruiz added a long two-run shot of his own to make it 7-4. A two-out walk to Jacob Young compelled the Mets (45-45) to bring in closer Edwin Díaz, whose wild pitch let Young score from second base before he struck out all-star CJ Abrams to end it.

After Nimmo’s home run, Irvin settled in by retiring the next six batters. In last week’s matchup, he relied heavily on his curveball (40 pitches) and his fastball (38), mixing in 18 sinkers, two cutters and a slider. But Tuesday, his game plan was different: He threw 31 sinkers, 30 curveballs and 29 fastballs (plus three change-ups and one cutter).

“He wasn’t as sharp,” Martinez said. “His breaking ball wasn’t as sharp. He fell behind a couple of hitters, made some mistakes just by location. All in all, he gave us some innings that we needed. It’s just one of those days.”

In the sixth, Irvin faced the same problem he grappled with in the second. He recorded the first two outs but this time allowed a single to Bader to bring up the top of the order. Lindor crushed a 2-1 fastball that Irvin left over the heart of the plate for a two-run homer. The Citi Field lights flashed blue and orange as Irvin turned to the home-plate umpire and asked for another ball.

“He’s been a workhorse for us, so it’s one game,” Martinez said. “He has one more start before the [all-star] break. He’ll lock it in and go back out there and compete.”

Notes: Joey Gallo has not been cleared to run the bases but is partaking in light baseball activities nearly a month after he strained his left hamstring. Over the weekend, Gallo fielded groundballs at first base. On Tuesday, he took swings in batting practice. …

Right-hander Trevor Williams is continuing to rehab from a right flexor strain that has sidelined him since the beginning of June. He’s throwing from 60 feet but has a ways to go until he can throw off a mound. Gallo and Williams entered the season as potential trade candidates, but their injuries make it unlikely they’ll fetch much of a return — if any — with the trade deadline looming July 30. …

Left-hander Jose A. Ferrer is scheduled to throw Tuesday for Class AA Harrisburg as he rehabs a left shoulder strain. After that, he and righty Cade Cavalli (Tommy John surgery) will head to Florida to throw during the all-star break.