The word Sir Alex Ferguson used was "shattered". The last time Manchester United were humbled so comprehensively by Manchester City, a 5-1 defeat at Maine Road in September 1989, Ferguson went home and lay for hours with his head buried under a pillow. This 6-1 defeat will certainly have hurt even more. "I can't believe it," he said. "It's our worst-ever day."

United have not lost this heavily at home to City since 1926. One more goal for Roberto Mancini's team would have equalled United's worst home defeat, inflicted by Newcastle United the following year. As for Ferguson, his record books have been scarred forever. "It's the worst result in my history, ever," he said. "Even as a player I don't think I ever lost 6-1. It's an incredible disappointment."

Mancini tried to play down the achievement when he was asked whether United's first home defeat in 38 games represented one of the more satisfying results of his career. "No," he replied, "I'm satisfied because we beat United away and I don't think there are a lot of teams that could win here. But in the end there are only three points – we don't take six points."

But this felt like far more than just another win. Darren Fletcher described United's performance as "naive" and Ferguson accused the 10 men of being carried away by United's history of improbable comebacks.

"It was hard to believe we were 1-0 down but that's retrievable. The sending off [of Jonny Evans] was the killer blow. After that, we kept attacking. It's all right playing the history books but common sense has to come into it. When we went to 3-1, 4-1 we should have settled for that. We kept attacking and we should have just said: 'We've had our day.' But our two full-backs were playing like wingers. At times it was two versus three at the back. And that was suicidal, crazy."

Mancini had generous words for the beaten side. "This is important for our confidence but we should appreciate the mentality United have. United are too strong for this [to affect them]. They know, like me, there are only three points and this is only one game."

United, he said, are "still one yard above us. We can change this only after we win the title. After that, maybe it will be different but, for now, United are still better than us."

Mancini could afford to laugh at Mario Balotelli's latest self-inflicted controversy, the striker's house catching fire after he and four friends let off fireworks from a bathroom window in the early hours of Saturday.

"If we want to talk about Mario as a football player, I'd put him in the first five players in the world," City's manager said. "The problem is his age. He is young and he can make mistakes.

"As a player, he is incredible and I hope for him, and for football in general, we arrive at the day when Mario has completely changed his mentality because, after that, he becomes one of the best three players in the world, like [Lionel] Messi, like Cristiano Ronaldo."

The message Balotelli revealed on his T-shirt – "Why always me?" – after scoring the first of his two goals summed it up. "It's Mario," Mancini said. "He's crazy. But I love him because he's a good guy. I don't know what happened with the fireworks. The good thing is he was not hurt. But all I know is that he is sleeping at a hotel now."

The day ended in angry scenes outside Old Trafford, with missiles thrown at the car taking the Glazer family away from the stadium.

"We have to recover," Ferguson said. "In the history of Manchester United this is another day and we will recover. But that kind of defeat will make an impact on the players. There's a lot of embarrassment in that dressing room and quite rightly so."

Ferguson once derided the modern-day City as being "all talk". Now he has hard evidence that Mancini's team are the most serious threat to United's title hopes. "People call us the noisy neighbours," Micah Richards, the City right-back, said. "Well, here we are."