LOS ANGELES — Buster Posey has made no secret of the fact that he believes there’s a way to win consistently at Oracle Park. He grew up in this game with Tim Lincecum, Matt Cain and Madison Bumgarner, and he has talked repeatedly in recent months about the depth of young starting pitching that the Giants once again have.

On a seven-minute call with beat reporters on Sunday night, Posey brought it up again. He believes in building around pitching, and he believed in left-hander Kyle Harrison. But he also saw an opportunity that was too good to pass up.

It was going to hurt in some way to acquire Rafael Devers, and the Giants had to part with a 23-year-old who is about 15 months removed from being widely considered the best left-handed pitching prospect in the game.

“Kyle Harrison is a guy that has tremendous upside,” Posey said. “He’s shown this year that he’s 95-98 (mph). I’ve been very outspoken about what I believe the foundation of success is for an organization: It’s pitching and defense, so to give up a guy like that is not an easy thing for us. Obviously, the Red Sox and (lead executive) Craig Breslow value that type of arm, as well.”

Harrison was pulled off the field just minutes before what would have been his fifth start in place of the injured Justin Verlander. The right-hander will be back from pec discomfort on Wednesday, but there was no guarantee that it would have been Harrison who would have returned to the bullpen. Behind closed doors, the Giants were discussing whether he should stick in the rotation and another young starter should be removed.

The discussion now will be about how to get through the second half, when Landen Roupp likely will hit an innings limit. The Giants figure to lean on left-hander Carson Whisenhunt, and his strong year in Triple-A made it easier to pull the trigger on parting with Harrison.

Whisenhunt is a top-100 prospect as well, but few Giants in recent memory have reached the level of hype that accompanied Harrison. He was a top-25 prospect in the minors and twice pitched in the Futures Game, and for a while, the Giants thought he would be the long-term partner for Logan Webb.

Webb and Harrison are close, with the ace becoming a mentor for the young lefty. On Sunday, Webb said he was excited to add a top-10 hitter to the lineup, but admitted the trade was bittersweet. He spoke with Harrison in the clubhouse and later said he relayed a simple message: “Go be a star over there.”

“You guys have heard me talk about Kyle before. I’m the biggest believer in him,” Webb said. “He gets to go learn from some really good pitchers. You’ve got (Walker) Buehler over there, (Garrett) Crochet, (Brayan) Bello. You’ve got (pitching coach) Andrew Bailey over there.

“I think it’s a good landing spot for him and I think he’s going to be great. I told him I’ll be a friend for life.”

As Harrison departed, Webb asked if he would still be a neighbor in the Scottsdale area. Harrison moved there as a prospect so he could train near the team’s facility all winter, but his future now is up in the air. He told Webb he wasn’t sure what’s ahead of him, but the immediate move from the Red Sox was to option him to Triple-A, which will prevent him from facing the Giants this weekend at Oracle Park.

At some point soon, though, the Red Sox will slide him into their rotation. Harrison was said by those close to him to be happy with the opportunity to grow with a young core of talented position players, and short-term, the trade will come with an unexpected perk. His younger brother, Bear, is a talented Texas A&M catcher who will spend the summer in the Cape Cod League, playing only about 90 minutes from Fenway Park.

It’s been a chaotic few months for Harrison, who got a late start to camp, but the Red Sox are hopeful he can build off his recent success. He had a 4.56 ERA for the Giants, but there were flashes of his past dominance in May, and his fastball was back into the mid-to-upper nineties.

If Harrison can develop a consistent breaking ball, that part of the trade might come back to sting the Giants. They based just about their entire 2020 draft around Harrison, a De La Salle alum who was taken in the third round but signed well over slot as a way to keep him in the Bay Area and away from UCLA.

Farhan Zaidi thought Harrison would be a star, and it was easy to see that path when he debuted as a 22-year-old in 2023 and struck out 11 Cincinnati Reds in his first start at Oracle Park. The Red Sox had been eyeing Harrison for a while, too, and while the Giants didn’t want to part with him, there was no way to acquire Devers without including a marquee young pitcher along with Jordan Hicks, prospect Jose Bello and 2024 first-rounder James Tibbs III.

The deal brings a franchise cornerstone back to San Francisco, but leaves the Giants without much hitting depth in the minors behind top prospect Bryce Eldridge. And it leaves them without Harrison, who had hoped to partner with Webb, Hayden Birdsong, Roupp, Whisenhunt and others for years to come.

“I think there was a shock factor to it,” Webb said. “He grew up in the Bay Area and it sucks when you’re with a team for your entire career, close to family, and you’re getting ready to go face the best team in baseball, the best lineup in baseball, and you get pulled off the mound right before. But he handled it well.”

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