BOSTON — Imagine that the Boston Red Sox invited you to Fenway Park to choose their lineup. For each championship team in this century, you would not have needed any help finding the leadoff man.

The slash-and-dash lefty, Johnny Damon, would have been the obvious choice in 2004. Same for the feisty little second baseman, Dustin Pedroia, in 2007. The game’s best base stealer, Jacoby Ellsbury, looked the part in 2013, like the do-it-all dynamo, Mookie Betts, in 2018.

In 2021? Well, a lot is different about these Red Sox, who thumped the Houston Astros, 12-3, in Game 3 of the American League Championship Series here on Monday. They had to play a wild-card game for the first time. Their rotation is deeper than Houston’s, but not nearly as strong as it usually is in October.

And their leadoff hitter is Kyle Schwarber.

At 6 feet tall and 229 pounds, Schwarber is built more for the home run derby than the Boston Marathon. He has never stolen a base for the Red Sox. Originally a catcher and mostly a left fielder, he was so clumsy around first base in the division series that he raised his arms in triumph after making a routine play.

Schwarber does have a high on-base percentage — .343 for his career, one point better than Ellsbury’s — but that is not the reason the Red Sox bat him first.

“It’s not analytics, it’s not a hunch,” Manager Alex Cora said after Game 3, when Schwarber’s grand slam powered Boston to a two-games-to-one series lead. “It’s just out of necessity, to be honest with you. It feels really good right now with this lineup.”

In eight playoff games, that lineup has produced 57 runs and clubbed 20 homers while batting .317. Schwarber has led off in six of those games (he bats second against left-handers), part of a chain reaction after J.D. Martinez sprained his ankle during the final game of the regular season.

Cora had been using the left-handed Rafael Devers in the cleanup spot, with the dangerous Martinez behind him. Without Martinez for the wild-card game, Cora moved Devers to third and protected him with Xander Bogaerts, the slugging shortstop, at cleanup.

In that configuration, though, Schwarber could not bat second, because Cora wanted to split up his left-handed hitters. So Schwarber went up top, Kiké Hernández hit second, and everything seemed to fit.

“He’s a complete hitter,” catcher Christian Vázquez said of Schwarber. “He gets walks, he grinds at-bats. He gets a lot of pitches in the at-bat, and I think that helps us to see more, and get more information on the starter or the reliever. And he can go deep. A perfect pickup for us.”

Schwarber was recovering from a hamstring injury when the Red Sox acquired him from the Washington Nationals for a Class A pitcher on July 29. He spent two more weeks on the injured list before his Boston debut.

“It was like, for Christmas, you’re on vacation with your kids and everything, and all the gifts are under the tree — but they’re back home,” Cora said. “We knew he was going to have an impact, but we had to be patient.”

The Red Sox fell out of first place in the A.L. East as they waited, but Schwarber’s arrival sharpened their plan of attack at the plate. He hit .291 with a .435 on-base percentage and a .522 slugging percentage, drawing more walks in a month and a half with the Red Sox (33) than his former Chicago Cubs teammate, Javier Báez, drew all season for the Cubs and the Mets (28).

“It’s a different approach for us,” Cora said. “We were very aggressive the whole season. We were expanding. We didn’t walk too much, and when he got here and he started playing, it was different. It’s a different at-bat and other guys have followed his lead. And right now, this is the best I’ve seen this team this season, offensively.”

None of Schwarber’s teams has known quite where to put him. He has made between 100 and 125 starts at four different spots in the order: first, second, fifth and sixth. He said he had never led off until college, at Indiana University, and then only briefly. The Cubs tried him there twice as a rookie, in 2015, and more regularly two years later.

“My first go-round with it, I was fighting that conception of what the leadoff spot is and what you’re supposed to be, besides making it your own,” Schwarber said. “You don’t have to have that prototypical speed — not speed; yeah, I’m fast — but you don’t have to be that prototypical try-to-work-a-walk, or something like that. For me, I can do that already, so why would I try to force that besides just going up there and trying to take my at-bat?”

The leadoff hitter, after all, is only guaranteed to bat first in one inning. In a stacked lineup like Boston’s, the No. 1 hitter is often in position to drive in runs. In the second inning of Game 3, teammates took turns seeing pitches and drawing walks, bringing up Schwarber with a 2-0 lead and the bases loaded against Jose Urquidy, who started him with three balls.

Schwarber rarely swings with a 3-0 count; in his regular-season career, he has faced 89 pitches in that count and gone 0 for 7 with 82 walks. But plate discipline, he explained, is not about working walks but aggressively seeking strikes to punish. And when Urquidy fired a fastball down the middle, Schwarber was not about to take it.

He swung hard and launched the pitch to right for his third home run of October, all in Boston. A grand slam by a predecessor — Damon, in the 2004 A.L.C.S. — had muted a stunned and sour crowd in the Bronx. This one sparked a frenzy at Fenway, where the fans pierced Schwarber’s eardrums on his happy romp around the bases.

“That gave me a headache, and I loved it,” Schwarber said. “I loved every second of it. This is what you live for, right? This is it. Obviously, we still have steps to be where we want to be. But to have an atmosphere like that, we feed off that, and it’s only going to help us be better.”

If things get much better for the Red Sox, they might win yet another championship. With yet another leadoff hitter, of course.