Even before he learned how to walk, Dodger Stadium has been part of Jack Flaherty’s origin story.

It’s where the new Dodgers pitcher first visited at 6 months old, and would return frequently alongside his mom, Eileen, sometimes as often as 20 games per year.

It’s where his early love for the game was planted and nurtured, putting him on a path to the big leagues that first began in Sherman Oaks Little League.

It’s also where — before becoming a first-round draft pick, one-time Cy Young Award contender, and resurgent veteran pitcher acquired by the Dodgers in a blockbuster trade last week — Flaherty produced one of the brightest glimpses into his big-league destiny.

On May 31, 2013, in the CIF Southern Section Division I championship game with his Harvard-Westlake high school team, he pitched a shutout while driving in his side’s lone run in a title-clinching 1-0 win.

“That day,” former Harvard-Westlake coach Matt LaCour said, “kind of exemplified who he was.”

Tyler Urbach (17) and catcher Arden Pabst (7) rush pitcher Jack Flaherty (9) after winning the Southern Section Division 1 championship at Dodger Stadium on May 31, 2013.

(Patrick T. Fallon / For The Times)

In some ways, it was the start of a journey that will come full-circle Friday, when Flaherty — whose 8-5 record and 2.80 ERA make him one of the most important pitchers on the Dodgers starting staff — will make his much-anticipated home debut.

“I’ll have to probably take a breath and gather myself,” Eileen Flaherty said this week. “Because, he’s been there before, but now he has a Dodger uniform on.”

Flaherty’s future first became clear when he arrived at Harvard-Westlake’s Studio City campus, quickly making an impression on the school’s burgeoning baseball program.

“He was kind of touted as, ‘Oh, we have this incoming freshman who is super athletic,’” said Boston Red Sox pitcher Lucas Giolito, a fellow Harvard-Westlake product who was two years ahead of Flaherty in school. “He was like, varsity-as-a-freshman type of talent.”

Only, early on, Flaherty’s most obvious talents weren’t on the mound.

While the right-handed pitcher had sharp command and decent — though hardly overpowering — velocity as an underclassman, his tools as an infielder initially looked more promising.

“Everyone thought he would be a position player,” noted Harvard-Westlake’s then-pitching coach Ethan Katz (who has gone on to become the Chicago White Sox’s major-league pitching coach).

“He was gonna be the shortstop for the next four years,” Giolito said.

In Flaherty’s sophomore season, however, two things changed.

First, Katz helped Flaherty develop his now-signature slider, watching in amazement as the teenager quickly honed the pitch.

“I would tell him every day at practice, ‘Pitching first. Make sure you come down and see me,’” Katz recalled this week. “When he developed his slider sophomore year, that’s when he really took off.”

Then, when Giolito (the team’s senior ace and a potential No. 1 overall draft pick) blew out his elbow earlier in the year, Flaherty was elevated in Harvard-Westlake’s rotation, emerging as a reliable sidekick to another future MLB star, current Atlanta Braves pitcher Max Fried.

“That was kind of the beginning right there,” Giolito said of watching Flaherty that season. “It was like, this guy is a good hitter and a good infielder, but there’s something special about what he’s doing on the mound.”

With Giolito and Fried in the pros as first-round picks in 2013, Flaherty’s profile as a pitcher only continued to explode. He added life to his fastball. He put more bite on his slider. And he began to refine his mental approach, shifting more of his focus primarily to the mound.

“I get my work ethic from my mom, but also from the way that we worked [at Harvard-Westlake],” Flaherty said. “The way we went about our business, the way that we worked, the way we stayed after [practice]. Everything was detail-oriented.”

In that environment, Flaherty flourished.

In his junior year, he went 13-0 with a 0.63 ERA, earning National Player of the Year honors from Maxpreps while helping lead Harvard-Westlake to the CIF Southern Section Division 1 title game.

The added bonus: The final is annually played at Dodger Stadium.

“We obviously wanted to play for a CIF Championship,” Flaherty said this week, sitting on a railing of the Dodger Stadium dugout. “But we knew, with that, came being able to play here, which was just an unforgettable experience.”

Indeed, 11 years later, Flaherty and those close to him still remember the day vividly.

In the morning, the pitcher struggled to focus on his finals in school. “I don’t think I did well on them,” he joked.

As the team took a bus to the game, Eileen and other parents gathered for a meal, superstitiously repeating their outfits from each of the team’s previous playoff games. “I wore a pink shirt, like, the whole time,” she laughed.

Minutes before first pitch, though, Flaherty sat in the dugout with a quiet confidence, seemingly unfazed by a crowd of several thousand around him.

“You see it in championship games all the time,” LaCour said. “A lot of guys at that age, when the crowd gets loud, big environments, they kind of fall apart. But there was no fall apart in Jack. You knew what you were gonna get. You knew he was gonna execute and attack.”

Attack, Flaherty did, racking up eight strikeouts, giving up just six hits and escaping jams in both the third inning (leaving the bases loaded) and the seventh (when his left fielder threw out a runner at home plate).

“That’s game over,” LaCour thought to himself after the seventh-inning play at the plate. With Flaherty on the mound, “that ain’t happening again.”

By that point, Flaherty’s bat had also given Harvard-Westlake a 1-0 lead, driving in the game’s only run on a full-count single in the third.

“I was in Florida rehabbing from Tommy John [surgery], and I watched a live stream on my laptop,” said Giolito, who’d been drafted by the Washington Nationals the previous year. “I was fist-pumping and cheering. My roommate was like, ‘What the hell are you doing?’”

Full pandemonium followed the final out, which not only clinched a Southern Section title for Harvard-Westlake, but also a National No. 1 ranking from both Perfect Game and Baseball America.

“Four or five years before he got there, that program was a team that won like two games a year,” Katz said. “For the program, it was significant. It put Harvard-Westlake on a bigger map.”

Same went for Flaherty, who surged up draft boards during an equally dominant senior season in 2014 (he finished his high school career on a 23-game winning streak as a pitcher), before being drafted 34th overall by the Cardinals, giving Harvard-Westlake three first-round pitchers in three years.

“My time there was very instrumental,” Flaherty said this week. “Just in the way I continue to go about my business.”

Friday won’t be Flaherty’s first trip back to Dodger Stadium since. In his breakout 2018 season with St. Louis, he struck out 10 in a six-inning, one-run start. As a Cy Young candidate the following year, he spun seven scoreless innings while fanning 10 again.

Flaherty’s most recent trip was less memorable, a five-run clunker last April amid a career-worst season.

But this year, the veteran pitcher has regained his old, familiar dominance, leading him on a nostalgic road back to Chavez Ravine.

“It was in the back of everybody’s mind that the Dodgers would be buyers and the Tigers would be sellers, and hey wouldn’t that be cool,” said LaCour, who is now Harvard-Westlake’s athletic director. “So I think everybody’s looking forward to this weekend, and see him wear the white uniform at the stadium.”