Barcelona defender Gerard Pique has labelled his side's 2-1 defeat to Ajax in the Champions League as "a disaster."
Thulani Serero and Danny Hoesen gave the hosts a 2-0 lead at half-time, with Xavi's penalty early on after the break proving a mere consolation as Ajax held on to keep their hopes of progression alive.
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Although Barca, who host Celtic on matchday six, are already through to the last 16, Pique admits their hopes of claiming a fifth Champions League title will fade if they replicate their performance from the Amsterdam ArenA.
"We played a horrible first half," he said. "There is no excuse, it was a disaster. What happened in the first half can't happen again. The attitude wasn't right, the whole team know that."
Barcelona produced a typically dominant display in the second half, but Carles Puyol conceded a first-half lapse meant Ajax deserved all three points.
"Ajax were a lot more intense than us," he added. "In the first half they were far better, and you can't give one half away in Europe."
Ajax coach Frank de Boer was buoyant following the match, with his side now able to secure progression if they can beat AC Milan in their final match of the group.
"I don't think Barcelona expected this, we have achieved a victory thanks to our fighting spirit and our quality," he said. "The first half was fantastic, after that it was unfortunate that we were down to 10 men.
"I had said that Ajax would be the first team to beat Barcelona with the football we always want to play, which is offensive football."
De Boer was left to lament a red card to defender Joel Veltman, who was given his marching orders after taking down Neymar in the box.
"It was an expensive lesson for Veltman," he added. "He should have trusted on his goalkeeper and not have made the foul. After his sending off it became difficult and dangerous but we showed our lion's heart in this match."