It could have been poor form, low morale, tactical issues, or mentality. In short, Barcelona were out of sorts throughout the entire match.
One moment that summed up Barcelona's mental state came in the 85th minute.
Barcelona's attack broke down and Leeds United recovered possession.
When De Ligt received the ball in his own half, he saw Milinkovic-Savic pushing forward and launched a long pass.
Milinkovic-Savic, with his back to goal, used his body to hold off Piqué and flicked a header behind him.
The touch from Milinkovic-Savic sent the ball into the space behind Barcelona's defense.
Mbappé broke in diagonally at top speed like a cheetah hunting its prey.
At that moment, Busquets had already dropped into the gap left by Piqué to cover, and Roberto was tight to Mbappé. But for some reason, when Busquets swung to clear, he made a fatal error.
He misjudged the bounce and completely missed his clearance.
It was unbelievable.
As Mbappé surged through with Roberto in tow, he was just as surprised. He immediately stabbed the ball toward the penalty spot and accelerated to get on the end of it.
Lenglet reacted quickly. Just as Mbappé caught up and was about to shoot, Lenglet stretched a toe and poked the ball away, sending it toward the left side of the box.
But Mbappé, Roberto, and Lenglet had all collided by then.
Milinkovic-Savic arrived before Piqué, controlled the ball with his foot near the left corner of the area, noticed Cillessen rushing out, and calmly aimed for the far corner.
The ball skimmed under Cillessen and headed toward the right side of the goal.
Just as it was about to cross the line, Alba slid across the face of goal and hacked it away.
The ball cannoned off Alba's leg and looped up, but not far, dropping on the right side of the six-yard box.
João Félix, the Portuguese teenager who had replaced Dembélé less than ten minutes earlier, happened to be free in that unguarded pocket. He cushioned with his chest, then swung his right foot and shot immediately.
At that moment, two Barcelona players were inside the goal, two more stood on the goal line, and including Cillessen there were five bodies in and around the line.
But after Milinkovic-Savic's effort, they had all lost their balance and could not react.
Félix's strike flew straight into the right side of Barcelona's net.
"GOALLLLLLLLLLLLLLLLLL!"
"The fourth goal!"
"Still for Leeds United!"
"My God, Leeds United really played crazy at Camp Nou tonight!"
"Barcelona's performance has been unbelievable. They don't look like La Liga leaders at all, more like a relegation side."
"Leeds United have gone wild with their attacking football at Camp Nou."
"The fourth goal is from João Félix, the talented Portuguese forward, only 18 years old."
"Pochettino probably didn't expect this. He brought Félix on at 3-0 just to let him feel the rhythm and atmosphere. For young players, playing at Camp Nou is rare."
"But unexpectedly, he picked up a loose ball and broke Barcelona's net."
"Truly surprising. I bet even Leeds United didn't see that coming."
"From the slow-motion replay, you can see Barcelona's defensive problems clearly. Busquets made a basic defensive error."
"Barcelona's players fought hard, but Leeds United's attacks were simply overwhelming."
"4-0!"
"Gao Shen's team have once again created painful memories for Barcelona, even though Gao Shen isn't the coach this time."
…
Sitting in the Camp Nou stands, Gao Shen was stunned for a moment when he saw João Félix score. He could not help but stand and applaud the team's performance.
"Wow! Beautifully done!"
Around him, Lucas, Borrell, and the others also rose and cheered. Only UEFA President Ceferin and Football Director Boban stayed composed. After all, this was Camp Nou, and their positions were sensitive.
When Gao Shen sat back down, Ceferin and Boban leaned over to congratulate him.
"Your team's performances this season are still remarkable," Boban said with a smile.
Leeds United's rise in the Champions League began in 2016-17, when their young side reached the semifinals, equaling the club's best European record. Many thought that was Leeds United's ceiling.
Especially after that summer, when Leeds United sold a number of players, including Felipe Anderson, now at Barcelona.
Manchester City even signed three key men, Mahrez, Cancelo, and Benjamin Mendy.
At the time, almost everyone thought Leeds United's fairy tale had ended.
Unexpectedly, in 2017-18, Leeds went even further, winning the Premier League and reaching the Champions League final, stunning everyone.
This summer, Leeds United sold core players again, yet their level did not drop.
Especially with this 4-0 away win at Barcelona, the aggression and attacking intent, the energy and sharpness on the pitch, made this team unforgettable.
"The most important thing is youth development," Gao Shen said with a faint smile.
Ceferin and Boban both nodded.
Leeds United's academy is well respected across Europe.
Every year, UEFA runs the UEFA Youth League in parallel with the Champions League group stage. Leeds United have won multiple titles in that competition.
While youth results do not tell the whole story, they prove that Leeds United's academy is excellent.
It is also why Leeds United have consistently produced new stars in recent years.
For example, Mbappé and João Félix, who just scored.
Of course, it has also brought controversy. Leeds United often sign prospects from elsewhere.
They recruit from France, Portugal, and even across the UK.
On one hand, excellent youth development, on the other, aggressive recruiting. Together they have created the debate around Leeds United today.
"I heard from Sir Alex Ferguson that you bought Leeds United to prove you could run a successful, financially healthy club without relying on money. Is that true?" Ceferin asked with a smile.
Gao Shen laughed quietly to himself.
He had indeed said that, but was it really possible?
Leeds United succeeded because he knew he was a transmigrator. With his connections in European football, he was able to sign players from everywhere and build a strong Leeds United and academy in just a few years.
If it were anyone else, it would be hard to replicate.
But he could not say that out loud.
Gao Shen only smiled and nodded. "Transfer fees and wages have been soaring in recent years, putting huge pressure on clubs. Weekly salaries of four or five hundred thousand pounds are now common. It is outrageous."
Ceferin frowned and agreed.
"I've spoken with many clubs at the European Club Association. Everyone wants UEFA to step in and curb the trend, but honestly, it is difficult."
He paused, then continued, "People say I support sovereign wealth funds like those behind Manchester City and Paris Saint-Germain. Even funds from Saudi Arabia or Egypt are interested in entering. They think this will disrupt the landscape."
Gao Shen was a bit surprised that Ceferin brought it up himself.
"It's true, but I disagree. Will they really stay out? A lot has changed since the Americans entered and began making arrests. Football is no longer as independent as before. We need to consider more factors when making decisions."
"We all feel salaries and fees are getting out of hand, but is there a solution? Do you think our financial audit teams are better than those at Manchester City, Paris Saint-Germain, Real Madrid, or Manchester United?"
"Impossible," Ceferin shook his head firmly. "You are smart, Gao. We could increase investment massively, but the cost would be astronomical. Even if we did, there is no guarantee audits would find anything. The staffing and outsourcing alone would be huge expenses, and they would have to be deducted from Champions League revenue."
"More importantly, they will not listen to us. We are not law enforcement. Clubs can register companies in the Cayman Islands or anywhere. Real Madrid, Bayern Munich, Manchester United, even Leeds United, are global companies registered in Europe. We simply cannot regulate you effectively."
In short, UEFA can only check the financials submitted by clubs. They cannot fully monitor the true state of club finances.
Listed clubs like Manchester United are a bit more transparent, but for entities like Real Madrid and Leeds United, oversight is limited.
Without full financial oversight, there is no way to truly control wages and transfer spending.
Because there are always ways to pay those costs.
Not even the United States can fully regulate companies like Apple or Google.
The EU has been talking about fining Apple and Google as well.
In essence, global enterprises cannot be regulated perfectly, in any country or any industry.
Ceferin's point was that if regulation fails, introduce competition to curb excess through market forces.
From a broader perspective, Gao Shen felt Ceferin also wanted to apply pressure on the elite clubs, but the one thing he had not considered was limiting UEFA's own power.
Or perhaps he had considered it, but did not want to touch it.
Human nature.
(To be continued.)