Gus Quattlebaum grew up just north of Boston, and he remembers the 1986 World Series, so he knows all too well the way a postseason error at first base can impact the Red Sox and reverberate throughout New England.

That’s why, as the Sox vice president of pro scouting, Quattlebaum couldn’t get over what Kyle Schwarber did in the third game of the division series. One inning after he botched a routine play at first base — flipping an easy out high over the head of Nathan Eovaldi — Schwarber actively mocked himself right there on the field at Fenway Park, in full view of 37,000 screaming Red Sox fans. The next groundball hit his way, Schwarber fielded it cleanly and tossed to Eovaldi for the out, then he punctuated the mundane play with a wildly unnecessary celebration, pumping his fist and tipping his cap as teammates all around him howled with laughter.

“I’ve never seen anything like that in this city,” Quattlebaum said. “It was mindboggling, because we’re a city that wants to crush you, and he’s like, go ahead and crush me. … It was like Ray Bourque after a goal, for me. I couldn’t believe what I was watching when it happened.”

He was not alone. Schwarber has been a revelation in his not quite three months with the Red Sox. Not an obvious fit when the Red Sox traded for him — Schwarber was hurt and didn’t play the right position — he was their biggest upgrade, and arguably the most impactful offensive addition of any team at the trade deadline. The move was underappreciated at the time, but the Red Sox believed in Schwarber’s bat, his work ethic, his experience, and his attitude. They believed Schwarber would find a way to fit and make a difference, even if it wasn’t obvious when or how.

He’s done more than that. As the Red Sox face elimination at the hands of the Astros in Friday’s Game 6 of the American League Championship Series, Schwarber’s importance looms large — much larger than anyone might have anticipated — as a pivotal difference-maker on and off the field. He’s been a vocal leader in the clubhouse, a willing student at first base, and a tone-setting menace at the top of the order, and if the Red Sox are to survive and advance to the World Series, Schwarber will almost certainly play a significant role, either with his bat, or with his leadership and example.

“We were very aggressive the whole season,” manager Alex Cora said. “We were expanding. We didn’t walk too much. And when he got here, and when he started playing, it was different. It’s a different at-bat, and other guys have followed his lead.”

Before Schwarber arrived, the Red Sox were in trouble. July was their lowest run-scoring month of the year, and as the July 30 trade deadline approached, their front office identified a left-handed hitter as their most glaring offensive need. Ideally, they wanted a left-handed hitter that could play first base, but that market was limited. According to multiple sources, the team talked to the Cubs about Anthony Rizzo, but at some point, Chicago largely disengaged and seemed to move on, perhaps already sold on the package that eventually would send Rizzo to the Yankees.

Early in the process the Red Sox also had identified Schwarber, even though a left fielder was not a perfect fit positionally.

“We felt we had enough versatility on the roster to make just about anything work if we got the right bat,” general manager Brian O’Halloran said. “Honestly, with Schwarber, we saw him as one of the best (bats) if not the best bat on the market — left-handed bat, certainly.”

The Red Sox expressed that interest to the Nationals, and when the Nats eventually decided to sell, they circled back to the Red Sox on July 26 — the Monday before the Friday deadline — to put Schwarber on the table. Their general manager, Mike Rizzo, has a reputation for getting to the point, and he let the Red Sox know his asking price was 20-year-old starting pitcher Aldo Ramirez, who had a 2.03 ERA in Class-A Salem. The Red Sox considered Ramirez one of their best pitching prospects, a high price to pay, but perhaps not an unreasonable one.