Premier League - Ferguson: My worst day ever
A stunned Old Trafford crowd saw the champions concede six goals at home for the first time in the league since 1930, and Ferguson offered a damning assessment of his team's performance.
"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. That's challenge for me too."
United had Jonny Evans sent off early in second half when the score was 1-0, then capitulated in astonishing fashion as City plundered three goals in the dying minutes,
"I can't believe the scoreline. The first goal was a blow for sure but it was retrievable at 1-0," said Ferguson.
"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," he said.
"It was a bad day."
The result saw City extend their lead at the top of the Premier League and go five points clear of United.
However, Ferguson insisted United would bounce back from the disappointment.
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.
"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.
"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."
City manager Roberto Mancini, however, paid tribute to United's continued attempts to attack despite the game being out of reach.
He said: "I have more respect for United because after the sending off they continued to try and play and score. It is an incredible mentality."
Mancini added: "I think that after the sending off it was easier but I think we were playing very well.
"The first 25 minutes United played well but there was no chance. When you come here, United are a strong team, and you can have some problems.
"It's an important three points, we played well here. There will be not be many teams that come here and come to win."