Clockwise, from left: Mets pitchers Nolan McLean, Brandon Sproat and...

Clockwise, from left: Mets pitchers Nolan McLean, Brandon Sproat and Jonah Tong. Credit: Jim McIsaac; AP/Jeff Dean; Newsday/J. Conrad Williams Jr.

PHILADELPHIA – Have you heard the one about Cerberus?

He's a three-headed dog of Greek myth, and his job was to guard the gates of hell. That might not seem like it has a whole lot to do with the Mets, but when they finally leave Philadelphia Thursday night, the metaphor becomes apt.

See, the Mets entered Wednesday’s game against the juggernaut Phillies utterly battered – all but mathematically eliminated from winning the division, with the red-hot Giants 2 1/2 games behind them for the third and final Wild Card spot.

And what awaits them in Flushing? Why, no one other than old friend Jacob deGrom, starting Friday for the Rangers.

But…Cerberus.

Jonah Tong, Brandon Sproat and Nolan McLean will start against the Rangers, manager Carlos Mendoza confirmed Wednesday – a trio of pitching prospects literally tasked (perhaps unfairly) with guarding this team from October irrelevance.

Oddly enough, the three couldn’t be more different. McLean, with his three-quarter arm slot and baffling spin rate, comes off as something of a maverick. Tong, the north-south pitcher with the Tim Lincecum-like delivery, is affable and quirky. And Sproat, with the triple-digit fastball, is earnest and understated.

And yet, there's a common denominator.

“No matter how many times I get knocked down, if I get back up swinging, eventually I’m going to get good,” Sproat told Newsday.

“Mentality is a huge thing in this game, so when you maybe have a little lack of confidence, the velo might be the same but it might not play the same," he said. "When you’re confident behind your pitches and throw them with conviction, they tend to play better.”

Mets’ player development has focused on the trait, Mendoza said – both ensuring that their pitchers have plenty of secondary options, and the confidence to throw them. It’s translated on the major-league level, where all three have at times noted that they didn’t have their best stuff, but adapted.

For Mendoza, that’s been the most impressive part of the pitching experiment the Mets cannot afford to fail.

“You’ve got to give not only the players credit, but the people in player development that prepare those guys for situations like this," Mendoza said. "When you’re not feeling your best or you’re not having your Plan A or B, you’ve got to go to Plan C or D...We’re not just talking about stuff. It’s [about] how they’re handling themselves.”

In short, the kids have their pocket aces, but they also know how to play bad cards well.

Part of it has come by embracing what they can’t control, and trusting the process that got them to baseball's biggest stage.

Sproat learned that lesson first when he struggled in college and again earlier this season, when he posted a 5.95 ERA over his first 15 starts with Triple-A Syracuse.

He leaned on his faith and his folks and, frankly, himself.

“No storm lasts forever – that’s a promise from God himself,” he said. And then he realized something else: “I was trying to be a little too perfect in this game. You can’t be perfect in life.”

It's said perfection is the enemy of good, and it turns out that holds particularly true when throwing a nine-inch projectile to a man who can hit it 430 feet.

Interestingly, Sean Manaea – nine years Sproat’s senior – mentioned the same thing after his start Tuesday. He labored for the first two innings, giving up four runs, and then realized a freeing, if somewhat depressing truth: “It can’t get worse."

At that point, you might as well just let it ride, he said.

“It’s a letting go of all these things I want to do and just going out and pitching,” said Manaea, who went on to pitch three scoreless frames.

There’s a how and why these three players have been able to bring this maturity to the major-league level.

A lot of it is makeup: The organization has been impressed with their mental fortitude and problem-solving ability, and no doubt looked for those traits. They all have a ton of weapons, and buck the trend of chasing velocity for velocity’s sake (though Sproat does throw very hard). Mendoza credited the Triple-A staff, too: Manager Dicky Scott, pitching coach AJ Seager, and Eric Jagers, the Mets’ vice president of pitching.

“There are a lot of conversations before the game, a lot of conversations that happen during the outing,” Mendoza said. “It’s just sitting down with them and reviewing some of those outings and then going, OK, how can we help you get there?…There are a lot of people working behind the scenes to help these guys. At this level, [the hitters] are tough, but you’ve got to be able to make adjustments.”

Because in baseball, it’s not ‘if’ you get punched.

“You’re going to get punched,” Mendoza said. “You’ve got to be able to get back up.”

It’s a lesson the rest of this team needs to embrace if, 1. They want to make it to the playoffs and, 2. They want to last more than two games.

But at the very least, they’ve got Cerberus holding the line.

Notes & quotes: Tylor Megill is consulting more doctors after getting an MRI on his arm Monday, Mendoza said. Megill, rehabbing from an elbow sprain, experienced “tightness” in his arm in a game with Triple-A Syracuse Sunday and was shut down. Tommy John surgery has not yet been discussed, Mendoza said…Jesse Winker (back) is officially done for the season, but still doesn’t have a diagnosis, Mendoza said.

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