Mancini has regularly compared his team unfavourably to United, only for them to pip their neighbours to the Premier League crown last season.
He denies that was a tactic to motivate his players and insists City, who trail United by three points at the top, will be the underdogs at the Etihad Stadium tomorrow.
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 edge in a race that was 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."
Van Persie has added considerably to United's attack since his £27million summer move from Arsenal. City's strikers have failed to match last season's standards, but Mancini claims missing out on Van Persie does not bother him.
The Italian said: "No. In this moment, it is in the past."
City, who hope to have playmaker David Silva back after a hamstring injury, go into the derby on the back of more European disappointment.
Their dismal campaign in the Champions League came to a conclusion on Tuesday as they were beaten by Borussia Dortmund to miss out on a Europa League place.
There is a feeling their failure to make the competition could benefit their domestic title defence, but Mancini is not ready to admit that.
He said: "If you play Europa League you don't have time to recover. You play Thursday night and after the game travel, then you play Sunday.
"This can be difficult but now we are upset for this (to be out)."