The Ligue 1 giants appeared to be well on course for a quarter-final berth in Europe’s premier club competition after securing a memorable 4-0 victory over Barca in the first leg of their last-16 tie.
A return date at Camp Nou was considered to be formality by many, with PSG having revelled in their humbling of a continental giant.
Their taunts were to come back to haunt them, though, as Luis Enrique’s recorded a 6-1 win on home soil to complete one of the most remarkable comebacks in football history.
Mathieu played no part in that contest as he watched on from the stands, but the French defender has been quick to point out that his team-mates were motivated for the game by a lack of respect shown to them by their opponents.
He told RMC: "They're young and I think they lacked humility.
"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. 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.
"Even the wives of the players, such as Marquinhos', who put a photo of her showing four fingers with her dog. 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]."