Christian Pulisic might be the standard-bearer for the United States World Cup team, but it is Brenden Aaronson who has captured hearts in England.LEEDS, England — The stadium was already half-empty, and the steadfast few who remained inside Elland Road were in an unforgiving mood. Leeds United had just been beaten, again, this time by Fulham. A fourth defeat in a row, a seventh game without a win, and the specter of the Premier League’s relegation zone were starting to exert a dreaded, inevitable gravity.As the team made a perfunctory tour of the field, thanking the fans for their forbearance, they were met — mostly — with silence. When Jesse Marsch, the team’s American coach, followed a few seconds later, even that veneer of cordiality disappeared. He had been taunted by the crowd during the game. Now, he was being booed.At that point, Brenden Aaronson would have been forgiven for deciding to slip away to the dressing room. Few would have noticed that Aaronson, a 22-year-old American, had not been part of the players’ gloomy procession.Aaronson, though, did not take the easy way out. Instead, he walked slowly, deliberately around the field in Marsch’s wake. In front of all four grandstands, he held his hands up, open-palmed, as if begging for forgiveness. And, as he did so, the mood changed. By the time Aaronson left the field, his self-imposed ordeal over, the silence — if not quite the gloom — had lifted. Even in defeat, Aaronson had brought the fans to their feet.Whether accidentally or by design, Leeds United has spent much of the last three years as English soccer’s great thought experiment, a laboratory for challenging deeply held assumptions.The first hypothesis it tested was whether the outré methods of the Argentine coach Marcelo Bielsa, the sport’s most unapologetic ideologue, could work in the Premier League. The supposition had long been that no, they could not. Leeds gave Bielsa the chance to disprove it.He led the team to a ninth-place finish in his first season after winning promotion from the Championship and then plunged it into danger of relegation the next, but the adoration he earned from a fan base that tends toward cynicism was enough to overturn the established logic: At least one other English club is now toying with the idea of employing Bielsa.Leeds’s next challenge was, if anything, even more fraught. Leeds replaced Bielsa, in February, with Marsch, who became only the second American coach to take charge of a Premier League team. A few months later, he was joined not only by Aaronson — a native of Medford, N.J. — but by Tyler Adams, acquired from RB Leipzig but raised in upstate New York. Fair or not, how Leeds fared would be pitched as a referendum on English soccer’s attitude toward Americans.The results, thus far, have been mixed. Adams has been a steady, subtle success: a diligent, astute defensive midfielder, sufficiently well liked for a vast portrait of him to be hung from the imposing, cantilevered roof of Elland Road’s Jack Charlton Stand. “I didn’t realize it was quite so big,” Adams said after seeing it for the first time. “It’s pretty cool.”Aaronson has not been a starter for the U.S., but his game-changing cameos are a valuable tool for Coach Gregg Berhalter.Patrick T. Fallon/Agence France-Presse — Getty ImagesThe verdict on Marsch has been more contested. He earned some credit for steering the team clear of relegation last season, and an early season win against Chelsea in August, But that was followed by a string of disappointments in which Marsch’s team kept, as he put it, “finding ways to lose,” and a recurring theme emerged in the critiques of him: Leeds’ executives, and Marsch himself, noted that his nationality always seemed much more relevant after defeats than in the glow of victory.There has been no such ambivalence about Aaronson. He might have been a relative unknown when he arrived at the start of the summer from Marsch’s former team Red Bull Salzburg as a vaguely underwhelming replacement for Raphinha, the Brazilian wing who was then on his way to Barcelona.Aaronson may not have a regular starting role in Coach Gregg Berhalter’s United States team at the World Cup. In just three months, though, he has established himself as the great American success story of this Premier League season — ahead of even Christian Pulisic, now consigned to the ranks of replacements at Chelsea — and erased every last shred of skepticism that accompanied his arrival.Unlike Marsch, Aaronson’s Americanness does not appear to be a problem. He had already earned a song in his honor within a few weeks of arriving at the club, a reworked version of Estelle’s “American Boy.” “The Square Ball,” an ironic and occasionally acerbic Leeds fanzine and podcast, has taken to referring to him — affectionately — as the “Yank Badger.”The sobriquet hints at the source of his popularity. Under first Bielsa and now Marsch, Leeds has grown used to a style of play that borders on the physically exhausting. Both coaches demand that their players run. The fans have come to expect it, too. And even in a team marked by its (occasionally inefficient) industry and (occasionally counterproductive) intensity, Aaronson’s work ethic, his endless scurrying and snuffling, stands out.Aaronson came on as a substitute his team’s World Cup opener against Wales.Pedro Nunes/ReutersIn a victory at Liverpool in October that most likely saved his manager’s job, for example, Leeds not only ran more than any team had in any Premier League game this season, but Aaronson ran more than anyone else. He registered 8.2 miles, more than any player has run in any league game this year.“Brenden Aaronson loves grass,” Daniel Chapman wrote earlier this season in “The Square Ball.” “Green grass. Yellow grass. Part synthetic grass. All the grass, he loves all the grass, loves running in it, rolling in it, being on it, dancing across it, eating it up metaphorically with his running feet and perhaps literally with his hungry mouth.”Marsch regards that characterization, while not incorrect, as a touch reductive. “He has more quality than people think,” the coach said. “He’s a good finisher, he’s really clever with how to put passes together in tight spaces. It’s so much just about his ability to make final plays, and slow himself down a little bit in the final third.”Even Marsch, though, could not quite resist the lure of making a horticultural analogy. “He’s like a weed,” Marsch, a former M.L.S. coach with the Red Bulls, told MLSsoccer.com’s “Extratime” podcast earlier this season. “You almost see him grow before your eyes.”That is what has endeared him, so quickly, even to Leeds’s most hard-bitten, weather-beaten fans: not just his effort, but his intent. It is what has filled American fans with optimism about his contributions heading into Monday’s World Cup opener against Wales.That day against Fulham, Aaronson had no reason to apologize. The defeat, most certainly, had not been his fault. He had been Leeds’s best, and most effective, player. Still, though, he made his way around the field, still moving, even after the final whistle, still believing he could have done more. More