Unai Emery’s side celebrated wildly after their 4-0 thrashing of the Catalans at Parc des Princes, confident that qualification for the quarter-finals would be a mere formality.
Several squad members made triumphant social media posts. Adrien Rabiot and Layvin Kurzawa revelled in victory on Rabiot’s Instagram account, with Kurzawa displaying the 'V for victory' sign.
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Even Carol Cabrino, the wife of Marquinhos, joined in, uploading an image of herself holding up four fingers in the wake of PSG’s victory.
Mathieu has now suggested that such triumphalism not only angered his Barcelona teammates, but inspired them to turn the tie on its head and complete an incredible 6-5 aggregate comeback.
“They're young and I think they lacked humility,” he told RMC Sport. “In football, things change quickly, and it came back to haunt them. You have to wait for the end of both 90-minute matches.
“There wasn't only them [Rabiot and Kurzawa]. You can be happy to win 4-0, but you have to stay humble. A little message is fine, but there is still a second leg. You mustn't get carried away.
“When you have gone through to the next round, you can put out a few little things, but for me it was a lack of respect. I can tell you the dressing room was very angry about that.
“Those are the little things that really annoyed the dressing room and that's why we really, really wanted to pull off the remontada [comeback].”
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