Paris St Germain were "emotional and aggressive in a stupid way", contributing to their second-half capitulation in Wednesday's 2-1 Champions League defeat at home to Manchester City, former Arsenal manager Arsene Wenger said.
PSG looked sharp and focused in the first half of the Champions League semi-final first leg and took the lead when Marquinhos headed home from Angel Di Maria's corner.
But they lost control of the game to a transformed City side after the break, conceding two goals and receiving a red card in the space of 14 minutes.
Frenchman Wenger said PSG were guilty of retreating into their shell.
"I must say in the second half PSG were protective and not progressive any more," Wenger told beIN SPORTS. "Is it because they were not physically capable?
"After 1-1, they collapsed completely. They become emotional and aggressive in a stupid way."
Wenger said PSG's patchy domestic form - the French champions lie in second spot, one point behind leaders Lille with four games to play - had affected their confidence.
"What was interesting is that you could see that when it was 1-1, PSG mentally collapsed," Wenger added.
"This is the consequence of the team losing eight games in the league. This has an impact.
"When it's 1-1 the team knows they have lost many games, they carry that. It's not like the season before when they were unbeatable in the league.
"You could see the confidence dropped considerably and the mental impact when Man City equalised, for the semi-final of a Champions League, was too big."
PSG host Lens on Saturday, before travelling to Manchester for the return leg against City.
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