Mancini regularly compared his team unfavourably to United - and even wrote off their title chances at one point - only for them to pip their neighbours to the Barclays Premier League crown last season.
He denies that was a tactic to motivate his players and insists City, who now trail United by three points at the top, will be the underdogs at the Etihad Stadium on Sunday.
Mancini said: "We can't change the history. We are here (for) only two years, playing together, and they are used to playing this game for the title race for 20 years.
"This is normal when you play against a team that won everything in the last 15 years. You can't change this in two or three years and you need more time. For this reason, they have a small advantage."
When asked specifically about this season's title battle, Mancini said: "In this moment United are favourites but the season is long.
"They were a strong team and (then) they bought (Robin) Van Persie and (Shinji) Kagawa. They put another player that scores 25 goals into the squad."
Last season's Manchester derbies had a significant bearing on the destiny of the title. City's remarkable 6-1 win at Old Trafford gave them an extra edge in a race that was ultimately settled on goal difference. Their victory at Eastlands in April then pulled them level on points at the top with just two games to go.
Had United won that night, City would have fallen six points behind. That is a scenario that also faces them this weekend, albeit with much more of the season remaining.
Mancini said: "They can lose - they have three points more. We have three points less and we can't lose. But it is a derby, everything can happen, like last year. I think they are better than us in this moment but a derby is a derby. It is not the season because we have seven months after but it is important."