Although the memorable 6-1 win at Old Trafford and Vincent Kompany's winner at the Etihad practically handed Manchester City their first Premier League title in 44 years, the Italian says Sunday's clash will be different.
The Blues head into the clash three points behind their rivals at the top of the standings, but are unbeaten on home soil for almost two years.
And Mancini said: "We need to forget what we did last year because it doesn't matter now. But remember if we play like last year, we have a good chance to win."
The City boss also insists he is quite happy to settle for another single-goal win - despite United's defensive problems.
"A 1-0 is enough for us," he said. "They (United) will improve their defensive face. At the start of the season, we had the same problems and we improved. It will be the same for them.
"With the 6-1, they had only 10 players for most of the second half. But in the second derby, we played very well.
"We did not concede any chances. The second was a true derby because when you have 10 men, it's clear that you have some problems."