"They can lose. They have three points more. We have three points less and we can't lose," the Italian told reporters on Friday.
Champions City, unbeaten in 15 Premier League matches this season, won both encounters with their neighbours last season en route to a first league title in 44 years, including a 6-1 defeat of United in the first clash.
However, Mancini said United were always favourites because of their pedigree over the past two decades.
"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.
"In this moment United are favourites but the season is long."
Mancini highlighted United's purchase of Arsenal striker Robin van Persie as a big momentum shift before the current season got underway.
"They were a strong team and then they bought Van Persie and [Shinji] Kagawa. They put another player that scores 25 goals into the squad," he said.
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