WASHINGTON -- During the Mets’ workout on the eve of Thursday’s 2-0 Opening Day win over the Nationals, manager Mickey Callaway made a last-minute switch to the itinerary. Typically, veterans hit in the first group of batting practice, but Callaway shifted Pete Alonso, Dominic Smith, Luis Guillorme, J.D. Davis and Tomas Nido -- five position players who entered camp on the team’s roster bubble -- to Group 1.

As those five finished taking their hacks, Callaway called them aside and told them the last flight to Syracuse would leave in less than two hours. He paused, for effect. Then he said none of them would be on it.

“I got misty-eyed,” said Alonso, who became the seventh rookie to make his Major League debut on Opening Day in Mets history. “Everyone was like, ‘Don’t cry, Pete!’ I was wearing sunglasses at the time, thankfully. But yeah, I was beyond excited. I can’t really explain it in words. It’s just the most incredible feeling in the world.”

“He had this look on his face,” Callaway said. “The best way I can describe it is, I’ve never seen this look before. Like, ‘All of my hard work has paid off.’ That was a pretty cool look. Up till the last minute, he’s so humble, he truly didn’t understand he was on the team until we told him. It was a great moment.”

So excited was Alonso that he woke up naturally at 6 a.m. the following morning. When he arrived at Nationals Park, Alonso immediately changed into a full suit of Mets gear, unwilling to linger in street clothes. On the lineup card was an immediate test: Alonso was slated to bat second against Max Scherzer. And while he finished 0-for-3 with two strikeouts against the three-time Cy Young Award winner, chasing a slider well out of the strike zone in his first at-bat, Alonso rebounded to single up the middle in his final plate appearance.

Afterward, the ball rested on a shelf in Alonso’s locker. He planned to date and sign it, and snap it into a case.

“I guess now I can say I’m really a Major League Baseball player,” Alonso said. “I’m part of the club now.”

With club membership, of course, comes a high degree of difficulty. For the foreseeable future, Alonso is slated to start nearly every day at first base, relegating Smith and Davis mostly to the bench. With Jed Lowrie unlikely to return from the injured list anytime soon, Alonso is primed to bat second in the lineup quite often, giving him ample opportunity to prove he belongs.

“It’s incredible,” Alonso said. “This is something I’ve been dreaming about since I was a little kid, and something I’ve been working for my entire life. For all the fruits of my labor to pay dividends, it’s incredible. I can’t explain it in words. If I had the words, I’d say it. But to me right now, I’m just so excited.”