But none of them is Girardi’s top bullpen weapon these days. That designation belongs to a 26-year-old right-hander who began the season as a member of the Class AAA Scranton/Wilkes-Barre RailRiders’ rotation, posted a 4.73 ERA in five starts there, got called up in early May and has proceeded to become, in roughly four months, arguably the most feared relief pitcher in baseball whose name is neither Kenley Jansen nor Craig Kimbrel.

The Yankees’ top weapon is Chad Green. He is one of the biggest reasons the Yankees are leading the American League wild card race and remain within striking distance of the Boston Red Sox in the American League East, and one of the biggest reasons that, should the Yankees make it to October, they will be a dangerous postseason team. Everybody knows by now how the game becomes largely a battle of bullpens in October, and in Green, Girardi would have one of those overpowering, multi-inning firemen that plays so well in the playoffs.

“He’s capable of going multiple innings, and the results have certainly been there,” Yankees pitching coach Larry Rothschild said when asked about Green’s October potential. “But we’ll worry about that when we get there.”

Entering Wednesday, only Boston’s Kimbrel (50.0 percent), the Dodgers’ Jansen (42.5) and Milwaukee’s Corey Knebel (41.8), among pitchers with a minimum of 30 innings pitched, have struck out a higher percentage of opposing batters than Green (41.7). And no reliever in the game has made more appearances of four or more outs than Green, who has done so 10 times (with nine of them lasting two innings or longer). Andrew Miller himself, who practically invented that role, has only made four appearances of four or more outs.

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Lefties, who are hitting .143/.200/.286 off Green this year, and right-handers (.132/.209/.232) have been equally useless against him.

How did Green go from middling starter — a guy who failed to crack the Yankees’ rotation out of spring training — to one of the best relievers in the game in the span of one season? Green will tell you it’s a matter of simple improvement in the location of his pitches.

“Something just clicked, really,” Green said last week. “I guess the location is probably a little better this year. But other than that, I’m not doing anything too different than I’ve done in the past. It’s just this year, my location with my pitches has taken me to the next step.”

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Green’s success this year has been due largely to a fastball that has gained roughly one and a half mph over 2016, when he was a starter — a common scenario when pitchers shift from starting to relieving and throw fewer pitches per outing. He is also relying on the fastball far more often: In 2016, he threw the fastball in 39.2 percent of all the pitches he made; this year, the percentage is 65.3.

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The game, Girardi said, “has been simplified” for Green since he began coming out of the bullpen. “He doesn’t need to be a four-pitch pitcher. He’s a two-pitch guy, and it’s been successful.”

Hitters of late are also reporting a curious phenomenon with Green’s fastball: It has the illusion of gaining a late kick as it approaches the catcher’s glove. Physicists would say such a notion is impossible, but they’re also not standing in the batter’s box watching it.

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“It’s hard to describe but it’s almost like a late kick,” Baltimore Orioles rookie outfielder Trey Mancini said. “It’s not just put on a clothesline. It has some movement at the end, and you can just miss a ball that you think you see well. I don’t know if it’s movement or what. I can’t put my finger on it, but it does have a little extra something that makes it tough to hit.”

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Rothschild ascribes the illusion to the deception in Green’s delivery. “To a hitter’s eye, it seems like it has a late kick,” he said. “It’s hard to define what it is, but hitters do pick him up late. He hides the ball pretty well and he’s got a quick arm. It looks like he’s just playing catch, but the ball pops out of there pretty good.”

With Chapman, whom the Yankees signed to a massive five-year, $86 million contract this season, having lost his closer’s job and seeming nowhere close to regaining Girardi’s trust, and with Kahnle having yet to replicate his early-season success with the White Sox since joining the Yankees, Green has ascended into a more critical role as the season has wore on, entering in the seventh and eighth innings in back-to-back appearances two weeks ago.

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But mostly, his role is to do what he did against the Cleveland Indians in the first game of a doubleheader Aug. 30. In that game, he entered in the sixth inning of a game the Yankees trailed by a run and threw 2 2/3 scoreless innings, recording seven of his eight outs via strikeouts — a major league record for outings of 2 2/3 or shorter. Though the Yankees still lost, it was an eye-opening performance against the defending AL champions.

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The Yankees have plenty of work to do to a clinch a spot in baseball’s 10-team postseason tournament, but should they make it, they may be able to rightly claim they have the best bullpen in the American League. And at the heart of that bullpen, improbably, is Chad Green.