Then something changed. After claiming 33 points from a possible 36, the club have taken just five from 18 since, winning just once in their last six league games. That alarming run of form continued last weekend with a second-successive home defeat in LaLiga, this time against Leganés .
It has heightened the importance of Saturday's fixture against Atlético at the Olympic Stadium (stream LIVE at 3 p.m. ET on ESPN+ in the U.S.
The spotlight is now on Barça's high line for the wrong reasons. Have they been found out? Has the magic worn off? How have Real Sociedad , Celta Vigo , Las Palmas , Real Betis and Leganés all managed to take points from them?
ESPN spoke to players and coaches from those teams who have enjoyed success against Barça in recent weeks to try and determine if the dip can be reversed, or if Flick is going to have to compromise on his tactics.
Barça, with the youngest squad in LaLiga, hit the ground running this season, winning their first seven league games. Defeat to Osasuna came after Flick rotated his squad, but the norm was quickly established with another four-game winning run in the league while Bayern Munich were among the sides dispatched in the Champions League with a 4-1 win on Oct. 23.
There was a clear pattern to Barça's success: They were intense, pressing better than they have done in years, and they won the ball back much higher up the pitch. They were more direct, too. They made fewer passes, but more progressive ones. That took advantage of one of this squad of players' major strengths: they are good when there is space.
LIVE ON ESPN+ (SELECTED GAMES) The hallmark of the team quickly became the high line, with the back four pushing towards the halfway line whenever possible. For 12 games, it was more or less executed perfectly by Jules Koundé , Pau Cubarsí , Iñigo Martínez and Alejandro Balde .
The offside trap, therefore, had already become a talking point, but it took on even more significance after October's Clásico . How would it fare against Mbappé and Vinícius Júnior ? The answer was remarkably well, although the margins were fine, with one Mbappé goal only ruled offside after a lengthy VAR review.
Asked how he had experienced those hairy moments when Mbappé (offside eight times in the game) or Vinícius (offside three times) had run in behind, Flick said the only problem was that play continued, forcing his defenders to run back, when the flag could go up earlier. "We always play with the back four very high," he said postmatch. "Because the referees don't whistle [for offside], it looks dangerous, but it's not dangerous. The only thing is that everyone has to run back; this is the point I don't like so much."
The offside trap continued to work the following week. Two Espanyol goals were ruled out after VAR reviews, adding to some remarkable statistics. Through 12 league games, Barça were catching teams offside an average of 6.92 times per game. At the time, through Europe's top five leagues, Brighton & Hove Albion ranked second at 3.9. Barça's previous highest per-game average, since data is available, is 3.16 in the 2022-23 season. The highest on record, through the course of an entire season, is Villarreal , with 5.55 in 2010-11.
"S--- November" The focus on Barça's high line had, perhaps rightly, become so exaggerated that it dominated pregame and postmatch analysis. The element of surprise under a new coach had gone and there was now a big enough sample size for teams to come up with plans to offset it. And that's what they did. It began with Real Sociedad's 1-0 win over Barça in San Sebastián early in November.
"We worked on their high line," La Real forward Take Kubo told ESPN. "We saw the [5-2] Champions League game against Red Star Belgrade and the first goal they conceded. I personally had the impression that the key is the secondary runs from the midfielders.
"I think those situations are difficult for them because they stop the line watching the striker or the wide men, but I am not sure they take into account the runs from the second line, the midfielders, even the defensive midfielder, because ours can run in behind as well. I think, based on this game, maybe they will think about how they defend, I don't know."
Barcelona's defence broke new ground with its offside trap earlier this season, but the spotlight is now on their high line for the wrong reasons. Joan Valls/Urbanandsport/NurPhoto via Getty Images That loss was no anomaly. Barça then dropped points at Celta, giving up a two-goal lead to draw 2-2, and were beaten 2-1 at home by Las Palmas. The Canary Islanders had worked all week on beating Barça's initial press and then making the most of the space behind their back line by using the pace of attackers like Sandro Ramírez and Fábio Silva .
"We had to take advantage of when we had the ball, push them back or try to push them back them into their half, which is where we had said it was more difficult for them, and then take advantage of the chances we had," Las Palmas midfielder José Campaña told ESPN. "When we had the opportunity to run in behind, we had chances.
"We didn't make the most of them in the first half, but our luck changed after the break. I think we played out from the back well for the first goal. We knew that with fewer touches we could hurt them the most and that's how we did it."
Defender Mika Mármol spoke about the importance of playing through Barça's pressing. "Throughout the week, we worked on bringing the ball forward and breaking out [from the back] against the pressure they apply," he told ESPN. "With their style of play, with the line really high, we wanted to take advantage of that and we were able to.
"We know we have fast players in attack that are good when there's space and that Barça have a high line. It is risky but it's worked well for them, but with the players we have up there we were able to hurt them."
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Those results led Flick to proclaim he was happy that "s--- November is now over," but performances have not markedly improved since. After beating Mallorca 5-1, they twice led against Real Betis before drawing 2-2, and then lost 1-0 at home to Leganés. Once again, teams targeted their high line. Mallorca's goal came via a deep run from full-back Pablo Maffeo , who caught Barça's backline flat-footed.
"Barcelona's defensive line as high as the halfway line is risky," Betis coach Manuel Pellegrini told ESPN. "They have created and conceded a lot of chances, with the high line snuffing out a lot of openings when it's done well. For me, it's a huge difference between the [back four] and the goalkeeper, but it's getting results so far because they're still going well in LaLiga and the Champions League.
"More than anything, [getting behind] it depends on the technical quality to be able to find those passes in between the defence and the goalkeeper. That's how to attack such a high line.
"Of course players, like Isco and [Giovani] Lo Celso , to name the creators at Betis, are players that can make the difference. They see the space and have to calculate the timing of the pass just right. That's talent, so rather than telling them how to play a pass, it's about preparing the plays and the movement of their teammates so they can find those passes."
High line to blame, or low energy? There have been glimmers of hope dotted between bad results. Barça have remained strong in the Champions League, beating the previously unbeaten Brest 3-0 and then becoming the first team to triumph at Borussia Dortmund this season, winning 3-2. The 5-1 win at Mallorca also showed that even if teams do deploy deep runners, and even if they do score a goal from it, it can still be hard to stop Barça when they click in attack.
"The idea was clear, but it's not easy because the temptation is there to run away from your marker and it's difficult not to fall into the offside trap," Mallorca coach Jagoba Arrasate told ESPN. "We did well with the goal we scored, but not so well on other occasions."
Mallorca midfielder Sergi Darder added: "If Barça play with that line, there's a reason for it. It's complicated [to break down]. It's easy to know what you have to do against a defence like that, but hard to actually do it.
"They encourage you to [play the ball in behind] and you're always offside. We knew never to give the pass on the back of the first movement. The problem is they press you, too, and you have to play the ball and if the timing is off, it's offside."
Sources at Barça did not want to reduce their poor run of form to the idea the high line has been found out, either, even as the number of times they catch teams offside per game has slipped from 6.92 to 5.83 in the space of six games. They say there are other factors. Flick, in addition to saying his young side will experience ups and downs, has highlighted the fact the team must defend as a whole. He has spoken about a "disconnect" between the attack and defence, the implication that a decrease in the intensity of the pressing has made it easier for opposition teams to attack Barça's backline.
There have also been games without star winger Lamine Yamal . Barça have not won any of the four league fixtures which he hasn't started this season. He missed matches with an ankle injury in November and is now out again for another four weeks with a similar problem. Robert Lewandowski 's form in front of goal has dropped off, too. Despite scoring 23 goals in 23 games this season, the 36-year-old striker has just one in his last four and missed good chances against Leganés, when he registered six shots with an accumulated xG of over 1.6.
The team's mental and physical freshness has also been questioned. Barça have been dependent on the same core of players and there has been little rotation. There is a lack of depth. Ten players have played over 1,500 minutes already this season in all competitions. Dani Olmo , limited by injuries, ranks 11th with 787 minutes and no one else has played more than 700 minutes in the squad.
It was a surprise, then, that only one change, Eric García for Cubarsí, was made against Leganés following the win in Dortmund. Flick's assistant coach Marcus Sorg later suggested, for the first time, that perhaps there was a tiredness element to Barça's performances.
"The last weeks were very hard but the data before the game was good," Sorg said in a news conference after the Leganés defeat. "It was the reason why we kept a lot of players from Dortmund on the pitch. But after the game, sometimes you know more than before."
Barça are still well placed, top of LaLiga and second in the Champions League standings. But with the winter break looming, Saturday's year-ending game against Atlético feels huge. Lose and they would slip three points behind having played a game more, all that good work earlier in the season unravelling. Maybe there is some danger after all.
ESPN's Gemma Soler and Moises Llorens contributed to this report
