Sir Alex Ferguson slammed his Manchester United players after suffering the heaviest defeat of his 54-year career against rivals Manchester City on Sunday.
Ferguson was left shell-shocked after City ran riot against 10-man United, finishing 6-1 winners to score their biggest win at Old Trafford in 85 years.
"It was our worst ever day," said Ferguson. "It's the worst result in my history, ever. Even as a player I don't think I ever lost 6-1."
"I can't believe the scoreline. The first goal was a blow for sure but it was retrievable at 1-0."
A furious Ferguson, who saw Jonny Evans red-carded, was unhappy that his team had not moved to staunch the flow of goals as City went for the jugular in the second half.
"The sending off was a killer for us. We kept attacking when we went 4-1 down and we should have just said: 'We've had our day."
"We just kept attacking. They were attacking three versus two. It was crazy football," Ferguson told Sky Sports.
"I thought with the experience we've got - Rio Ferdinand, Patrice Evra - they would (defend more) but we just kept attacking. Sometimes there has to be common sense about it. It was a bad day."
Ferguson said the scale of the defeat could also come back to haunt United at the business end of the season if goal difference comes into play.
Before the match, United had trailed City by only two goals in the standings. After Sunday's brutal mauling, that had stretched to 12 goals, with City now five points clear at the top of the table.
"What did concern me was the goals for and against. Goal difference may count. Last year it was in our favour, most years it is in our favour...this time maybe not."
Ferguson nevertheless vowed that his team would bounce back.
"We will react, no question about that. It's a perfect result for us to react to because there is a lot of embarrassment in the dressing room and that will make an impact," he said.
"We'll come back. By January we'll be okay. We usually get the show on the road in the second half of the season and that will have to be the case."
"We've played all the teams around us and they have all to play each other so the second half of the season is important to us now."
Manchester City keeper Joe Hart meanwhile attempted to keep a lid on the euphoria.
"We've got to stay level headed," he told the BBC. "We're a winning side and a team of winners but you don't get extra points by winning by that margin or by beating Manchester United."
"You could hear the fans today and we are delighted for them. We're a unit, the players, the staff, the fans and that is how we roll."