City head to in-form Newcastle on Sunday defending their status as Premier League leaders with just two games of the season remaining.
The Blues owe their position to Monday's victory over United at the Etihad Stadium.
But Mancini has to wind back a further three-and-a-half months for the point when he knew his side would not approach such a battle with an inferiority complex, even if it was a day that ended in defeat.
"For playing against United, the FA Cup game in January when we lost 3-2 was really important," he said.
"It wasn't the 6-1. That game was easy.
"But in the FA Cup, we had to play with 10 men for 80 minutes and we were also 3-0 down.
"Although we lost, they didn't have a shot in the second-half, even though they had 11 players and we had a chance to score the third goal.
"It changed our mentality against them."
It is why Mancini did not feel the need to summon up any Churchillian-type speeches on Monday as his team moved to within touching distance of City's first title since 1968.
"Normally, I don't say anything special in the dressing room," he said.
"I just say that the most important thing for us is to play football.
"Against United it was easy. I didn't say anything. The players knew it was a strong game and the last chance for us."
It is why, despite the enormity of the prize, Mancini does not believe his team will falter through pressure alone.
He accepts City may not win on Tyneside, just as they may fail in their final game against relegation-threatened QPR, either of which would United back in.
But Mancini is certain such eventualities would occur through the vagaries of life rather than any suggestion City had bottled it.
"That won't happen," he said.
"It happened six weeks ago. I don't know why. Probably because we had some important players injured. But it won't happen now.
"We can lose because in football you can lose every game. But I am sure we will play a good game."
And Mancini is taking a very cold-eyed assessment of the job that needs to be done.
City are presently the Premier League's top scorers. They also have the meanest defence and have done the double over United for only the second time in 42 years.
By any rational assessment, they have been the best team this season.
So far though, it means nothing.
And, just as he would doubtless shake his head at those City fans who claim they would be happy to win nothing else as long as this season's title was secure, Mancini is deriving little pleasure from what his players have done thus far.
"We are the best team in Manchester. To be the best in the country we need to win the next two games," he said with a smile.
"It is not important to beat United twice or beat Chelsea. The only important thing is to win the championship at the end."
Long-standing City fans find it hard to escape the theory that, having come so close, their heroes will now throw it away in some ludicrous manner.
After all, snatching defeat from the jaws of victory is part of their DNA and there could hardly be more tricky opponents this weekend that Newcastle, fresh from their win at Chelsea, in which Papiss Cisse cemented his status as buy of the season.
But Mancini bridles at such talk.
With an FA Cup, a Champions League campaign and now a title challenge behind them, the Italian does not believe "typical City" exists anymore.
"I don't think we are typical Manchester City now," he said.
"Over the last two or three years we have improved a lot and we are a good team."