Arsene Wenger angrily accused Arsenal of complacency after they slid to an embarrassing 2-1 home defeat to Hull City on Saturday.
The Frenchman could barely contain his fury at his team, whose proud unbeaten run at Emirates Stadium, which had stretched back to April 2007, was smashed by second-half goals from Geovanni and Daniel Cousin.
Defeat also ensured Arsenal were denied returning to the top of the English Premier League table, which is occupied by Chelsea, But Wenger's main concern was his team's approach to a game they had been expected to win at a canter.
"We weren't completely switched on to the level you need to be to win the game," Wenger said. "Human beings are not machines and perhaps subconsciously we thought we would just win it.
"We were a bit careless and it was a good lesson for us. We now know that if our attitude isn't right, we can lose games and Hull played very well, especially in defence.
"You can never afford defeats but it is very concerning to lose a game like we did today. We had a good chance to go top of the league but at the end of the day, if we had had the same level of concentration as Hull we would have won the game."
Wenger, who had seen his team forge ahead through Paul McShane's 50th-minute own goal, has urged the north Londoners to bounce back from this damaging defeat against FC Porto in Tuesday's Champions League group match.
"We responded well after losing at Fulham earlier this season and now we have to have an outstanding game on Tuesday night," he said.
"The dressing room is very shocked and quiet but that is part of mental strength, to deal with that and show how united we are.
"We have to repair the damage done psychologically because this loss was unexpected. It will be difficult to swallow for us. I am confident we will respond well because the disappointment is so deep.
"The overall spirit is very good. We were just caught against Hull by being too easy. That won't happen on Tuesday because Porto are always a difficult side to play."
For Hull manager Phil Brown this was a momentous victory, one that debunked at a stroke the theory that the best newly promoted teams can hope for against the Premier League elite is damage limitation.
It was also a reward for an attacking approach which saw the Tigers field two strikers, Geovanni and Marlon King, and maintained the club's superb start to the season, which has seen them gather 11 points from six games.
"We picked a team to come here and cause them problems," Brown said. "They're a great side but we went with Geovanni because we played positively against Everton the week before.
"We have a drive, an endeavour and a belief that we can come to places and get a result. We have a great group of players: we aren't fazed by anything because we have good experience in that dressing room and good talent. I wasn't surprised we won today."
Brown reserved special praise for Geovanni, whose remarkable goal, which was swerved into the top corner with his right boot from 25 yards, was a fair reward for a performance that buzzed with menace.
"I expected him to score from there!" he jokingly said. "He's a technically gifted player who can move the ball, like all Brazilian players. He has his own technique for it and it was far from a one-off for him. He's a magical player."