WASHINGTON — The joke, of course, is that any of this would be remotely surprising by now. The Mets have abided by the Harvey Rules from Day 1, have tread lightly around him, have allowed him the kind of leeway and latitude that should never be afforded someone with 75 career starts, no matter how promising he used to be.

So why wouldn’t he duck and run now?

Why wouldn’t he leave it for his manager and his teammates to answer for him, to speak on his behalf, after another humbling bell-ringing at the hands of the Nationals, another night when he was less Dark Knight than Pale Pawn, another night when he couldn’t recapture even a fraction of the old magic?

“We’ve got to think about not just what’s best for Matt but for the team,” Terry Collins said after this 7-4 drubbing was complete, after the Nationals had chased Harvey with five runs in the fourth and fifth innings, three of them on home runs. That made it 14 runs (11 of them earned) for the Nats off Harvey in 7 ²/₃ innings going back to last week.

“I’m surprised,” Collins said. “This guy’s way too good to continue like this.”

Collins was asked a few specifics about Harvey’s stuff and the manager said, “You’ll have to ask Matt that.”

Of course, by that time, Harvey already had bailed, already had left the ballpark, had refused to stand and be counted the way professionals do, the way athletes who want to be looked at as stars are required to. It was a terrible idea, one that any number of Mets officials — starting with Jeff Wilpon, who was here, and chief PR man David Newman, who was also here — should have anticipated and gotten out ahead of.

But Harvey has long played by his own guidelines, and has been enabled to do so, the Mets complicit in allowing that to happen. When times were good in 2013, and for most of last season, that meant playing the role of man-about-town, of big-stage bon vivant. It meant cultivating this image that he was a man’s man, a tough guy’s tough guy, equal to the demands of both stardom and the big city.

What a joke.

And what an empty jersey he’s turned out to be.

Look, the pitching is only one of his issues. It isn’t easy when the one thing you’ve been good at your whole life disappears on you. But you know what? Nobody ever said it was easy. You know who’s struggled in New York? Every star you can name, from DiMaggio to Reggie to Jeter, from Seaver to Piazza to Wright. And you know what they all had in common?

They were all grown-ups. They took their responsibilities seriously in good times, in bad times, when they were raking and when they were scuffling. They stood up, they shouldered their load. They may have hated it — nobody likes getting interrogated, nobody likes to be reminded when they’ve failed at their jobs — but they were man enough to stand up, take the heat, move on.

Harvey, he slipped out the back door.

It would be pathetic if it weren’t so predictable. This is who Harvey is, the ultimate front-runner, wanting everything his way when times are good and making the rules up as he goes along when they aren’t. His phony act hasn’t just worn thin, it’s worn out. The Mets are worried about his arm? They need to be worried about his heart.

“It’s not my job to straighten Harvey out,” Nats manager Dusty Baker quipped before the game. But he’s also a guy who has been around the sport forever, who has seen and experienced everything. He knows. More than anybody possibly can, he knows how things work.

“But from a player’s perspective, you don’t feel loved. You don’t think you have any friends. It’s a lonely place to be. People walk by you, they hold their head down. They don’t know what to say to you. You don’t look at the newspaper, you don’t look at TV. It’s a very lonely place to be.”

Of course it is. Again: If pro sports were easy, we’d all have a crack at playing them. They’re not. They come with personal sacrifice. They come with the responsibility of the job, and that means not dumping your troubles on the shoulders of everyone else while you make your way to the back door.

Harvey was lousy Tuesday night. He got beat with his third-best pitch, awful changeups to Ryan Zimmerman and Anthony Rendon, and with an indifferent fastball that Daniel Murphy hammered onto Capitol Hill, maybe doing a little extra pimping as he took a long gander at his handiwork. His fastball reached 97. But what’s clear is his arm isn’t the prime issue here.

His head is. His heart is. And his willingness to stand up and be counted — that absolutely is. You keep hoping he can figure it out, because when he was great back in 2013, and for stretches last year, it made whatever ballpark he was in crackle with hope and life and expectation. He’s having a rough go at pitching. That’s tough.

And an even harder time being a professional.

That’s inexcusable.