In the biggest moment of Josh Walker’s career, disaster struck.

The Mets rookie was pitching in only his third big league game Tuesday night against the Yankees. The stands were packed for an early summer rivalry game and the two New York teams were knotted at 6-6 in the top of the sixth.

Billy McKinney was on second base with no outs when Anthony Volpe sent a shallow fly ball to right-center field. Brandon Nimmo and Starling Marte both converged, but it was Nimmo’s ball and it’s a catch Nimmo typically makes. The center fielder is having an outstanding season on both sides of the ball and has made several dazzling plays to save runs.

But this time, it was too shallow and the ball drifted too far to the right. It went off his glove. It was an uncharacteristic mistake. The play was scored a double for Volpe and put runners on second and third. McKinney eventually scored the go-ahead run after the Mets went to the bullpen for Jeff Brigham.

Nimmo took it hard, but Walker, who took the loss, took full accountability.

“That pitch that I threw was supposed to be six inches higher than it was, so it probably would have gotten either weaker contact or a swing and miss,” Walker told the Daily News. “But Nimmo goes out there every day, and every time he’s going for the ball, he’s going hard and he’s trying to make a play. He doesn’t take a play off. He’s the only center fielder in the league I want out there. I don’t fault him for that at all.”

A few years ago, that loss might have stuck with Walker. It might have eaten at the Otisville, New York native. But the rookie has worked hard to change his mental approach in order to turn the page and learn from mistakes instead of dwelling on them.

By Wednesday, he had put it behind him.

“I would have let it affect me a little bit more because the end result didn’t work out the way I wanted it to,” Walker said. “In the past, I’d be upset about. Throughout my whole career, I’ve always just pointed the thumb, not the finger. That’s been a big lesson I’ve learned throughout my professional career. And now it’s helped me today to be ready to go again.”

This is important for Walker because he’s about to be given a big opportunity. The Mets will have to play their next nine games without right-hander Drew Smith, who is serving a sticky stuff suspension. He can’t be replaced on the roster and the team has to figure out how to absorb the loss without overextending an already taxed bullpen.

On Wednesday night, they had right-handed starter Tylor Megill ready to go in case Justin Verlander’s outing was cut short. But the Mets are going to need pitchers like Walker to step up and pitch big innings.

Walker has always had good breaking stuff, but his fastball velocity has gone up to the mid-90s this season. The Mets like that he’s unafraid to go right at hitters and attack the zone. He is 2-1 with a 1.45 over 14 innings with Triple-A Syracuse this season.

“I’m trying to be aggressive towards my target,” Walker said. “It’s knowing where I want to throw a pitch and just kind of attacking that spot and not overthinking my mechanics while I’m out there on the mound or even getting ready. It’s just trusting my stuff more and going out and giving everything I’ve got the tank.”

But the team also appreciates his resiliency. A 37th-round pick out of the University of New Haven in 2017, Walker battled numerous injuries throughout his career. While pitching for Low-A St. Lucie in 2019, his car was hit by a driver making an illegal turn near Clover Park and his pitching arm was injured once again.

Then came the 2020 COVID-19 shutdown. He didn’t pitch competitively. There were times he thought about leaving baseball behind forever, but he believed he could pitch in the Major Leagues and knew he would regret it if he gave up on his quest to get there.

“I had times where I was like, ‘Is all this work worth it? Am I just spinning my wheels? Should I start another career?’” Walker said. “I’ve had times where I’ve sat with myself and I’ve even broken down just thinking, am I making the right decisions? Is this worth it? But my biggest regret would have been not giving everything I had to try and make this happen.”

It’s easy to see why someone in his position might overthink things. And for many years, he did. It wasn’t easy for him to change his mental approach to baseball and build the confidence needed to succeed in the big leagues. But he’ll need every bit of that confidence now, because here’s the downside to Smith’s absence: The Mets have to utilize their pitchers with options, shuttling them in and out of the bullpen to keep it fresh and keep their late-inning relievers from getting overworked.

That means Walker could have a great outing and still get sent back to Triple-A the next day. Walker is better equipped to handle that now, knowing he could show enough to convince the Mets to bring him back and keep him around the next time.

“To not give it everything you have, I feel like that’s just like spitting on that opportunity,” Walker said. “Whether it works out or not, at the end of the day, I know I worked as hard as I could have to make it happen. Now that I’m here, I’m going to continue to do the same thing.”

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