LONDON -- When you watch Leeds United, it's sometimes hard to imagine that one of the most thrilling, dynamic and relentless teams in the country is managed by a gentle 64-year-old who lives in a flat above a shop in the small Yorkshire town of Wetherby, and sometimes holds coaching meetings in the local Costa coffee house.

Marcelo Bielsa almost walked away from Leeds at the end of last season. Their fading form in the final weeks of the campaign (stream Championship matches on ESPN+) and eventual defeat in the playoff semifinal to Frank Lampard's Derby County meant they missed out on promotion, and it wasn't a given that he would return for a second shot.

But return he did, to the joy of Leeds fans who adore him for obvious reasons, but the rest of us should be pleased he stuck around too. As they showed against Arsenal in their 1-0 FA Cup third-round defeat on Monday, they will not only be competitive should they reach the Premier League, but more importantly for neutrals, they will be thrilling to watch.

This game felt less like a cup tie and more like a rehearsal for Leeds, or perhaps more accurately a mass reassurance that their promotion campaign ultimately won't be for nothing. It's incredibly tough to predict which teams that come up from the Championship will thrive and survive in the top tier: take this season, for example, when last term's Championship winners Norwich are on course for relegation, second-placed Sheffield United are riding high and Aston Villa, who made it through the playoffs, are scrapping but keeping their heads above water.

- Stream a replay of Arsenal vs. Leeds United on ESPN+

Therefore you can't guarantee that, should Leeds hold on (they top the Championship, nine points clear of third place) they will be automatic successes, but you can guarantee that they will enhance the division and everyone should be delighted to have them.

Apart from every team they play, of course.

"They're a nightmare," said Arsenal manager Mikel Arteta after the game. "Playing against them is like going to the dentist. They're a great team. What they have built there is powerful."

Leeds should have been at least 3-0 up at the break. They had taken 15 shots to Arsenal's three and would have been out of sight if they had a better centre-forward than Patrick Bamford. They swarmed an Arsenal team that seemed surprised at how good they were, as if it hadn't occurred to them that a side in the second tier could be quite so dominant.

"I tried to tell them what to expect," said Arteta, who grew more and more agitated as the half progressed. "He shouted a lot," said Alexandre Lacazette about how the Arsenal coach responded at half-time.

From the front to the back of this Leeds team are players who will be, unless something goes wrong, stars in the top flight next season. Centre-back Ben White will play for England before too long. Many think Kalvin Phillips, their ice cool screen at the base of midfield, should already have done so. Jack Harrison is on loan from Manchester City and wouldn't look out of place in their squad. Mateusz Klich is a relentless presence in midfield.

And it's also worth remembering that this wasn't their full-strength team. Bielsa rested keeper Kiko Casilla and captain Liam Cooper, Stuart Dallas and Helder Costa were on the bench while playmaker Pablo Hernandez has been injured for a month. There were two debutants in the team, the 20-year-old midfielder Robbie Gotts and 19-year-old stand-in keeper Illan Meslier; both of them were absurdly composed for players so young making their first starts in such circumstances.

A goal felt inevitable as the first half progressed; it didn't come, but both home players and fans were rattled. Not long before half-time, Sokratis Papastathopoulos had to urge a section of the Arsenal support to calm down after they were unhappy with how quickly he was taking a free kick in his own half. Panic was close to setting in. You had to occasionally remind yourself that this was Arsenal facing a team from the Championship.

It helped that Arsenal were initially so passive, and that Granit Xhaka was having the sort of game that inspires harsh opprobrium from the stands. He committed three, maybe four fouls that could easily have earned yellow cards, gave the ball away liberally, was slow to react to passes and generally displayed such a panoply of calamity that you wouldn't have been surprised if his shorts had spontaneously fallen down.

Maybe it's a little unfair to compare the two sides. The first half was a game between a side who have known exactly what they are and what their manager wants from them for the past 18 months, against another who've had three coaches this season and have essentially started from scratch in the past few weeks. Leeds were playing the sort of football that Arsenal aspire to -- or should aspire to.

After the break, things returned to the natural order. Arsenal woke up to the fact they were playing a Premier League side in all but name, and started playing like the Champions League aspirants they should be. Reiss Nelson bundled home what turned out to be the winner, a scrappy goal that you would struggle to say they deserved at the time, but their second-half performance was much more of the calibre you'd expect.

But they weren't the team that most of those present will remember from this game. That was Leeds, who will almost certainly be back here next season. In some respects it didn't matter too much that they lost; it was just as important that they showed they could live with, and even better a team like Arsenal.

Afterwards, Bielsa insisted that he could not "draw conclusions from one match," and pointed to a 4-0 preseason defeat to Manchester United as proof that they won't necessarily be this good in every game against top-class opposition. But he did concede: "We showed we were able to beat our opponent."

As they filed out after the final whistle, defeated but happy, a group of Leeds fans sang, "Leeds are going up." And they did so with gusto, safe in the knowledge that if and when that does happen, they will look like they belong.