WASHINGTON — This is what he would be missing, if he stayed away: Nights like this. Games like this. Teammates like this.

Not 10 minutes after the Mets had finished off this unfathomable 8-7 comeback win — growing their NL East lead to six games, shrinking the magic number to 19 — a roar bled through the walls of the visiting clubhouse at Nationals Park, spilling into the hallway.

Kirk Nieuwenhuis — Kirk Nieuwenhuis! — was getting his boxing belt, emblematic of the daily hero. And his teammates — close to 40 of them stuffed in a space not designed for 40 — went bonkers.

Including the pitcher sitting in the corner locker, No. 33. Matt Harvey.

“We’re a fun group,” manager Terry Collins said.

“This is such a great bunch of guys,” Nieuwenhuis said.

This is what Harvey would be missing, this stretch run that every night seems to yield a fresh set of characters and a more improbable story line than the day before. Maybe it was good that he got such an up-close-and-personal reminder while he still has the chance to figure a way to be a part of it.

“So much fun,” Harvey admitted. “I love competing with this team.”

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He wasn’t good Tuesday night, even if he was sabotaged by the first evidence that Yoenis Cespedes really is from Cuba, and not Krypton. The Nationals found holes and gaps and touched him up for seven runs and eight hits in 5 1/3 innings, on what seemed certain to be a routine (if dispiriting) loss …

Until.

“I just shake my head,” Collins said, and he really was shaking his head.

It was 7-1 when Harvey trudged off the mound, and still 7-1 with two outs in the top of the seventh when the really strange things began to happen, when Washington pitchers couldn’t find the strike zone with a GPS, when Cespedes — back in his cape — restored the three runs he had surrendered overrunning a ball by creaming a three-run double down the left-field line, a ball whose exit speed was estimated at 7,377 mph, when Lucas Duda finally nudged in the tying run with a walk.

The Mets dugout suddenly sounding like the 7 train at rush hour.

“You gotta be kidding!” Collins heard himself say, not more than 10 minutes after he had all but conceded, after he was thinking about subbing out his starters to get them rest in a lost-cause game. No more white flag. No more concession. He heard his own voice again, telling nobody in particular: “We’re gonna tie this thing.”

They tied it. Then Nieuwenhuis — Nieuwenhuis! — untied it in the eighth with a blast to right-center off Jonathan Papelbon, and then Jeurys Familia danced through a few raindrops to close it out in the ninth. They won the damn thing, 8-7.

“This shows an awful lot about this team,” said Harvey, who has exited so many 1-0 and 2-1 games that wound up going sideways, costing him victories, and was now hand-delivered one of the most welcome no-decisions of his life. “It was so enjoyable to watch the guys come back.”

These moments are so fleeting, and so unpredictable, which is what makes them so precious, so priceless. Seasons like this don’t often come in bulk. Do they add up to the $200 million contract lurking in Harvey’s future? Only in a dreamer’s soul. And again: Nobody — honestly, nobody — is asking Harvey to lay down his life — or his arm — for the cause.

But you have to figure out a way to be a part of this, don’t you? Even a professional emotional eunuch like Harvey’s agent has to recognize the need in his client to be a part of this, especially nights like this one, when a room of professionals can turn into a giggling gaggle of high school football players who just won sectionals.

Hell, even Collins felt like a kid again, after thousands and thousands of games, after four decades in the sport, gushing, “I’m not sure I’ve been involved in a bigger win than that right there,” he said. And, yes, he was still shaking his head.

“I thought yesterday was pretty good,” David Wright said. “This was amazing.”

The Mets, after everything, want Harvey to be a part of this. They want him in every postgame clubhouse handing off the belt. They want him to have a turn or two with the belt. Surely he wants in. Surely he wants to be a part of this. He says he does. How could you not?