CLEVELAND — Harrison Bader was supposed to be the athlete in center field, and the guy running around the bases with his hair on fire — if there was any hair left from the heavy cut required in the transition from St. Louis Cardinal to New York Yankee.

Even though the 6-foot, 210-pound Bader was bigger than the 5-11, 195-pound Mickey Mantle, he was hardly a larger-than-life figure in Mantle’s position, on Mantle’s team. When acquired for Jordan Montgomery while sidelined with plantar fasciitis, his role was simple:

Get healthy. Catch the ball. And make a play or two with his legs on the bases.

But hit three home runs in the first four games of the Division Series, including a game-changer Sunday night that helped pull the Yankees from disaster’s doorstep and landed them in The Bronx for an all-or-nothing Game 5?

That wasn’t plausible. That wasn’t real. Bader’s father Louis, a Rockland County guy who raised his son in Bronxville, grew up a Yankees fan who counted Mantle and Roger Maris as his all-time favorites. He could not have fathomed this October power surge, because nobody could have fathomed it.

That’s OK; strange things happen in baseball all the time, especially in the playoffs. In Philadelphia, the 87-win Phillies upset the 101-win Braves in four. In San Diego, the 89-win Padres used a five-run seventh to stun the 111-win Dodgers in one of the most devastating upsets in modern baseball history.

And here in Cleveland, the offensively challenged Guardians scored three Game 3 runs in the ninth to force the favored Yankees to play Game 4 for their lives. When Bader stepped to the Progressive Field plate in the second inning, he had more home runs in this series than did Aaron James Judge, who, you might recall, hit 62 of them during the regular season.

With one on and one out and the Yanks already holding a 1-0 lead, Bader sized up the first pitch from Cal Quantrill — an 89 mph cutter — and absolutely crushed it toward left center while Quantrill spun around and threw his arms across his body. Bader had victimized the same Cleveland starter in Game 1, but this two-run shot was an entirely different animal.

This 429-foot homer gave the Yankees a 3-0 lead, and some breathing room on a night when it was awfully difficult for the visitors to breathe. Bader’s blast would prove to be the decisive development in a 4-2 victory that naturally qualified as the biggest of the season, if only because there would’ve been no more season without it.

“He’s electric, really, an electric player,” said Gerrit Cole, the ace who looked like 324 million bucks. “Impact player. Impact player. Got moxie, got baseball awareness, gets after the ball on defense. A lot of good things to say about that guy.”

Including the fact that he just joined Mantle and Bernie Williams as the only Yankees center fielders to have hit three homers in a postseason series.

What a stunning turn of events. Back when Montgomery was letting it rip for the Cardinals, and Bader was too busy rehabbing his foot to play for his childhood team, the trade didn’t look like one general manager Brian Cashman would be bragging about at cocktail parties.

But the Cardinals are long gone from the postseason, and Montgomery only gave them 2 ²/₃ innings in their two-and-done wild-card appearance. Bader, meanwhile, has grown into an invaluable Yankee, a hitter near the bottom of the order capable of doing top-of-the-order damage.

“It’s been awesome,” said manager Aaron Boone. “He worked so hard to get back, worked his tail off rehabbing, showing up in a walking boot. And right away trying to just endear himself in the clubhouse, and absolutely has. … He loves to play the game. The power showing up here in the postseason for us has been big.

“He really stepped on that ball tonight. I mean, that’s a bomb. It was huge for us to give us a little cushion there early.”

Bader is the first player to hit his first home runs as a Yankee in the postseason He credited his success to an approach he described as “just remaining present.” Cole told him in the dugout, “You just play each out, you play each at-bat, as just a singular event. And you just go from there and keep repeating it until the game is over.”

If Bader keeps repeating his home run swing, the Yankees will be flying to Houston on Tuesday to play in the ALCS. The center fielder said the Yanks “feel like we’re in the driver’s seat” on this return trip to The Bronx, a little more than five miles south of where he played high school ball at Horace Mann.

Asked how the dream of someday hitting October home runs for the Yanks matched up with the reality of doing it, Bader said: “You definitely visualize it. You have a dream about it. Yes, I was obviously a Yankees fan growing up. My dream has always been to play major league baseball.”

Of Monday and Game 5, he said, “It’s going to be a great day. I can’t wait to wake up tomorrow and get to it.”

And why not? There would’ve been no tomorrow for the Yankees without Harrison Bader on their side.