Sir Alex Ferguson described Manchester United's 6-1 thrashing by rivals City as his "worst ever day" at the club and promised: "There will be a response."
Mario Balotelli and Edin Dzeko each scored twice as City tore United - who had Jonny Evans sent off - apart.
Ferguson said: "It's the worst result in my history. The impact will come from the embarrassment of the defeat."
Sergio Aguero and David Silva also scored for City in United's biggest Old Trafford defeat since 1955.
Ferguson believes the loss - which ended their run of 19 consecutive home wins in the Premier League - was self-inflicted, and criticised his team's gung-ho approach.
"It was a horrible defeat but it was suicidal," added the United boss.
"Jonny Evans's sending off was a killer for us. With 10 men we kept attacking - it was crazy football and ended up being an embarrassment. We should have just said: 'We've had our day'.
"I believe you shouldn't bring down a man on the edge of the box. Evans is young but he should let Balotelli go through and see what he can do.
"I thought with the experience we've got - Rio Ferdinand, Patrice Evra - they would have defended more but we just kept attacking. Sometimes there has to be common sense about it. It was a bad day."
Despite the manner of the defeat, Ferguson insisted that United will recover and close the five-point gap that now separates them and City.
"We'll come back," he said. "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 all have 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.
"I can't believe the scoreline. Even as a player I don't think I ever lost 6-1. That's a challenge for me too."
For the latest updates and reaction to this story, read Sportsday Live. Have your say on Twitter via the hashtag #bbcsportsday.