LONDON (AFP) - Arsene Wenger described his Arsenal flops as naive and suicidal after their defensive meltdown allowed Monaco to clinch a 3-1 win in the Champions League last-16 first leg.
His side were widely expected to see off the French club and advance to the quarter-finals for the first time since 2010.
Instead, they are on the verge of a fifth consecutive last-16 exit following an incredible meltdown at the Emirates Stadium on Wednesday.
The second leg is on March 17.
In a post-match press conference, Wenger is usually composed enough to deflect questions that ask him to criticise his players.
But after one of the most chastening defeats of his 19-year Arsenal reign, he was unable to conceal his frustration.
Taking aim at the careless play of Arsenal's defenders and the needlessly gung-ho attitude of the rest of the team, he said: "It was a horrible night. We missed chances and were a bit suicidal defensively.
"Their first goal was unlucky with the deflection but the second and third were suicidal. We cannot give away the third goal like we did.
"The balance wasn't right and when we lost the ball, we were exposed. I was most disappointed by our defensive naivety. It looks like we lost our nerves and our rationality. The heart took over the head and, at this level, that doesn't work."
Wenger suggested his players may have taken victory for granted.
"I hope we weren't complacent but it looks when you have no sharpness that anything is possible. We rushed our game. The difference was the mentality, we were too impatient because this is a game of 180 minutes."
Arsenal now need a three-goal victory to complete a miraculous escape in the return at the Stade Louis II. But Monaco have not conceded three goals at home in any match this season and no team have lost a Champions League first-leg knockout tie at home by two goals and progressed to the next round since Ajax in 1969.
Monaco boss Leonardo Jardim said: "We analysed Arsenal beforehand and we knew they have been a lot stronger in the first half and struggle a bit after the interval.
"Football is an art, you need to know how to attack and defend. We are a very balanced side. It is difficult to score a lot of goals in the Champions League but, today, with the space Arsenal gave us, we were able to exploit it."