David Peterson of the Mets pitches in the first inning of...

David Peterson of the Mets pitches in the first inning of a game against the Boston Red Sox at Fenway Park on July 28, 2020 in Boston. Credit: Getty Images/Adam Glanzman

BOSTON — Officially, David Peterson did not find out he would make his major-league debut Tuesday until the morning of.

Unofficially, he could have anticipated such an occasion for at least the previous couple of days. The Mets told him they might need him that night, so be ready.

In reality, he had prepared for this moment much of his life. And it was worth it.

Peterson got the win in the Mets’ 8-3 victory over the Red Sox, allowing two runs in 5 2/3 innings. The 24-year-old lefthander, who was the club’s top draft pick in 2017, became the first Mets starting pitcher to win his debut since Steven Matz in 2015.

“This is one the greatest days of my life,” Peterson said. “This is something I wanted to do since I was a little kid. To go out there and make my first major-league start and we got the win, I couldn’t have asked for much more.

“Being in a historical place like Fenway, having the history with the Red Sox drafting me out of high school [when he passed on going pro to attend the University of Oregon]. It all came together. One of the best days of my life. I’ll never forget this.”

The Mets (3-2) again took advantage of Boston’s poor pitching. Robinson Cano reached base four times (two hits). Jeff McNeil had two hits and two RBIs. Amed Rosario flared a two-out, two-strike, two-run single to right in the third.

J.D. Davis dinged a 328-foot homer off the Pesky Pole in the fifth, and he learned later that the ball had specks of the famous rightfield foul marker’s yellow paint on it. He plans to give the ball to his dad.

“That’s a pretty cool little memory,” Davis said.

Peterson, though, was the star. His game and his career, however, got off to a conspicuous start. His first batter, Jose Peraza, lined his second pitch 105 mph off the Green Monster. Davis played the ricochet smoothly — a specific skill he practiced with outfield coach Tony DeFrancesco this week — and fired to second base to nail Peraza trying to stretch it into a double.

“Not the way I would’ve dreamt up my first big-league out,” a smiling Peterson said. “It was something I was able to laugh at and able to take a deep breath after.”

His second batter, J.D. Martinez, was credited with a two-strike check swing, a call that the Mets badly wanted to go in Peterson’s favor, according to the jeers from the visitors’ dugout. Peterson answered by striking out Martinez swinging on the next pitch (which he did again in the third).

Peterson largely settled in from there, scattering seven hits and two walks and striking out three.

He gave up one run in the third — when he allowed the Red Sox to load the bases with nobody out — and one in the sixth, when Kevin Pillar’s RBI double chased him. Drew Smith struck out Mitch Moreland to strand Peterson’s baserunner.

“He didn’t shy away,” manager Luis Rojas said. “That’s a tough lineup to navigate three times, and this kid maintained his poise.”

Peterson’s debut illustrated the unpredictability — and tenuousness — of pitching depth. Over the offseason, he was probably ninth or 10th in line for a spot in the Mets’ rotation. But then Noah Syndergaard needed Tommy John surgery, Marcus Stroman suffered a torn left calf muscle, Walker Lockett experienced back discomfort and Corey Oswalt pitched in long relief Sunday.

All of a sudden, Peterson was the man for the fifth game of the season, without the benefit of a minute in Triple-A, which isn’t having a season in 2020. And it worked out great.

Peterson even came away with four baseballs as souvenirs: his first pitch, his first strikeout and the ball he handed to Rojas when he got pulled, plus the last out of the game (courtesy of catcher Wilson Ramos).

“We definitely always talked about having him as depth for us and the future of our starting rotation,” Rojas said before the game. “The future came today.”