Two goals each from Mario Balotelli and Edin Dzeko and one each from Sergio Aguero and David Silva left City’s neighbours “embarrassed” according to their boss as Blues fans celebrated.
But City boss Mancini is looking further ahead and declared: “It isn’t important that it was 6-1, it’s important that we have beaten a fantastic team.
“It’s important for our confidence - it’s not easy to come here and beat United. Now we have one game less to play, we have three more points, and we have been to Old Trafford.”
City also boosted their goal difference - which could be a factor in a tight title finish - as the Reds succumbed after defender Jonny Evans was dismissed at the start of the second half.
Mancini admitted: “After the sending off it was easier, but I think we were playing very well even before that. The first 25 minutes United played very well though they didn’t really have a chance.
That’s normal at Old Trafford, United are a strong team and you expect to have some problems
“I’m happy for Mario, he scored two important goals, he played well and was serious out on the pitch.
“I have even more respect now for United - after the sending off they continued to play and try to score goals, and that’s an incredible mentality.”
Home manager Sir Alex Ferguson said: “It was crazy football at times. It was a bad day, our worst day. The sending off was a killer for us.”
