Mancini insisted he got it all wrong as Darron Gibson's first Premier League goal since March 2010 enabled United to move level on points with City as they crashed to a 1-0 loss at Everton.
Gibson's telling contribution for his new club ensures City have only won once in their last 21 visits to Goodison Park and completed an excellent evening for David Moyes, the Everton manager, who celebrated his 150th game in charge of the Merseysiders in style.
And the City manager was content to take on the chin a defeat that could have serious repercussions as United bid to beat their deadly rivals to the Premier League crown.
"We didn't prepare well and we had a problem. I can't make the same mistake next time," Mancini said.
"I'm very disappointed with my mistake and we didn't prepare well with the training. We are still at the top and there are 15 games to go and it's important that we win the game at home on Saturday (against Fulham).
"I thought it would be easier but it never is here. We didn't deserve to lose the game but it was because of my mistake that we did."
Moyes insisted this victory was one of the sweetest of his ten-year reign and he was also celebrating the £5.5m signing of Croatian international striker Nikica Jelavic from Rangers.
Jelavic has agreed a four-and-a-half year deal, with Everton striker Louis Saha set to join Spurs in a surprise move on deadline day.
"It typified everything Everton was about - and could be one of the best. It was a brilliant effort and it was certainly up there," he said.
"That's as good as it's been at Everton for a long time.
"We've got Jelavic and he's the centre forward we've been looking for for a while. It's great news for us and we just hope now he can help us score some goals.
"He's come from a great football club and is coming to another one. He really wants to play in the Premier League and Everton.
"Jelavic also wants to play for Croatia at the European Championships so hopefully that will keep his form good."