The Italian boss believes that his arch rivals are leading by a large margin due to other teams being too "soft" in their approach to matches against the North West outfit.
"No-one plays well against United because they only play with fear," ESPN quotes Mancini as saying.
"Every team that plays against United plays very soft because they think this game is difficult, they can't beat them.
"If you play strong against United, you can beat them, like you can beat us."
Tonight's clash is the first time that City have played at Old Trafford in the league since last season's 6-1 victory over United.