On Tuesday morning, when Mets manager Luis Rojas told David Peterson that he would make his Major League debut that night, there was no outward burst of emotion. No obvious elation. Stoic, Peterson simply thanked his manager and said: "I'm ready."

Such is Peterson's demeanor, both off the mound and on. If the Mets had any doubts about the benefits of that attitude, Peterson erased them throughout the first 5 2/3 innings of their 8-3 win over the Red Sox. He gave up two runs and nothing more, becoming the first left-handed pitcher in 18 years to last at least five innings in a big league debut at Fenway Park.

• Box score

"That's his first outing," Rojas said. "He handled himself like it was one of many outings he's had at the Major League level."

The only real inflection point where things could have gone awry occurred in the third inning, when Peterson loaded the bases with no outs on two hits and a walk. He then ran the count to 2-0 against J.D. Martinez, who owns the league's highest OPS against left-handed pitchers over the past five seasons. Combining Martinez's reputation with the fact that Fenway Park can be a nightmare for lefties, it was clear that Peterson was in the stickiest of jams.

• Bruce Maxwell, Mets nearing deal (source)

On the mound, he resolved not to give into Martinez with a fastball down the middle. Instead, Peterson came back with a changeup that Martinez was early on, followed by a 3-1 sinker that dotted the outer edge of the strike zone. With the count full, Peterson then threw a devilish slider that dove beneath Martinez's bat at the last moment.

The next batter hit into a double play, and Peterson was on his way. When Rojas came to remove him from the game in the sixth, he once again congratulated the rookie.

"This is one of the greatest days of my life," said Peterson, who struck out three, allowing seven hits and two walks. "This is something I've wanted to do since I was a little kid. To go out there and make my first Major League start and get the win, I couldn't have asked for much more."

Only after the game did Peterson show the first hints of emotion, talking about his experience pitching at historic Fenway Park. The Red Sox had drafted Peterson out of high school in 2014, but as a 28th-round pick, he opted to go to the University of Oregon instead. Within three years, he had molded himself into a first-round talent. The Mets snapped him up 20th overall in the 2017 MLB Draft.

The team then set about developing him into what they believe can be a mid-rotation starter. Generally successful in the Minor Leagues, Peterson took a leap forward in the second half last season, when, on a tip from the Mets' analytics department, he began throwing his slider -- the same one he used to strike out Martinez -- more frequently. Peterson leveled up again this spring, following an offseason of lower-body work, when he showed up to camp featuring a fastball several ticks hotter than he had ever had before.

It was enough for the Mets to include him in their 60-player pool for the 2020 season, and enough for them to turn to him when Marcus Stroman tore his left calf muscle a week before Opening Day. With Stroman sidelined for the indefinite future, Peterson now figures to be at least a short-term member of the club's rotation.

"You can't ask a guy to have a better outing than he had tonight," said second baseman Robinson Canó, who reached base four times in the victory. "He was filthy tonight. He was putting pitches where he wanted."

Thanks to Canó and J.D. Davis, who hit a two-run homer and made a fine defensive maneuver off the Green Monster to erase the first baserunner of Peterson's career, the lefty became the first Mets starter to win his debut since Steven Matz in 2015.

Peterson seemed acutely aware of that sort of history. Before leaving Fenway Park, he packed up four souvenir baseballs: his first pitch, his first strikeout, the ball Rojas took from him in the sixth inning and the final out of the game, which catcher Wilson Ramos saved for him. Peterson should have plenty of opportunities for more, both this season and beyond. With only Matz, Jacob deGrom and injured starter Noah Syndergaard under team control next season, Peterson will almost certainly be a part of the 2021 rotation -- provided he continues giving the Mets outings like this.

"His poise, once again, is something that stands out," Rojas said. "His matureness, how he's able to handle the scenario, that's a great thing."