Barcelona coach Hansi Flick conceded his side were not at Paris Saint-Germain's level as they suffered a 2-1 defeat at the Olympic Stadium in the Champions League.
Ferran Torres gave Barça an early lead but Senny Mayulu levelled before the break and Gonçalo Ramos struck a 90th-minute winner for the European champions.
The defeat was Barça's first in any competition this season, while holders PSG made it two wins from two in the Champions League.
"You can see it today, there's no point saying we're on the same level," Flick said in the post-game news conference.
"But I believe in my team. Today wasn't our best day, but we are working again to reach this level. Today we lost, but we want to come back.
"We could see the level of PSG and I'm 100% sure we can also play like this. It's about 90 minutes. The whole team has to defend, attack, use the spaces and be involved in the ball possession.
"You can see in PSG everyone wants the ball, knows how to use the space, play 1v1, 2v1... This is also what we have to learn, we have to get better."
Barça had gone into the game hopeful of proving themselves against a PSG side missing Ousmane Dembélé, Désiré Doué, Khvicha Kvaratskhelia and Marquinhos through injury.
Torres' opener had fuelled that belief but Barça, who were missing key players themselves in Joan García, Raphinha, Gavi and Fermín López, dropped off in the second half.
Flick was impressed by the manner of PSG's performance, given they were without their first choice forwards.
"The philosophy they have is fantastic, with young players coming in, high quality, speed with the ball," he added.
"They had fresh legs, you could see they increased the level of the team. For me, we have a lot of matches the last weeks and of course we have to learn about this.
"We started very well, but after 30, 35 minutes, it was like PSG controlled more the game. We suffered a little in the first half but also in the second half we don't have this control of the game.
"We didn't have the structure we have when we are on our highest level. This is necessary when in the Champions League, especially against an opponent of this quality in PSG.
"In the second half, you could see that some players are really tired. We took care about Pedri, Marcus [Rashford]. Of course Frenkie [de Jong] also, but he really gave his best.
"I think at 1-1, normally in this moment you have to defend better, have a better structure in the defence."
Barça had come close to earning a point before Ramos' late strike caught them out of shape.
"Of course we are disappointed," Flick said. "When you defend, 90 minutes, the 1-1, then also you have to play a little smarter, clever at the end of the match.
"This cannot happen that they make a transition and score this goal. This we have to learn. We have to have a better structure. We have to learn."
Barça return to LaLiga action on Sunday when they travel to Sevilla looking to strengthen their grip on the top of the table.