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."
Van Persie has added considerably to United's attacking armoury since his £27million summer move from Arsenal.
The Dutchman, who hit a remarkable 37 goals last season, has already struck 13 times for United.
The 29-year-old was reportedly of strong interest to City but opted for Sir Alex Ferguson's side.
City's own strikers have failed to match their standards from last season but Mancini claims missing out on Van Persie is not something that bothers him.
The Italian said: "No. In this moment, it is in the past, this.
"We have good players. If we improve this situation - in this moment we don't score a lot of goals - I am sure if we continue to work we can improve."
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 sorry conclusion on Tuesday as they were beaten by Borussia Dortmund to miss out on a consolation spot in the Europa League.
There is a feeling that their failure to make the second-tier competition could benefit them in their domestic title defence but Mancini is not ready to acknowledge that yet.
He said: "Probably in February, March (it) could be because the difference is if you play Champions League you play Tuesday and Wednesday and you have three or four days to recover.
"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)."
Despite being pitted in a tough group, the Champions League failure - in which City claimed just three points from six games - was a huge disappointment.
Mancini tried to make light of suggestions he is now under even greater pressure to defend the Premier League crown.
He said: "I don't know this. You need to ask him (owner).
"I am happy with my job in two years here.
"I think that when you work and when you beat a team - sometimes you can have also a difficult moment.
"You can't win all the games and sometimes you can lose important games like in the Champions League, but this is normal for being on the top for a long time.
"Also Ferguson won his first trophy after seven years and his first Champions League after 14 years. I have another 12 years to win a Champions League!"