City are attempting to close the 12-point gap at the top of the table, and manager Mancini believes a bad result for United could help swing the momentum of the title race in favour of City.
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"Football is strange for this," he told reporters. "Sometimes you lose a game, you don't know why. Maybe one, two, three games.
"We took two points from three games against Liverpool, Southampton, QPR [in late January and early February]. Sometimes this can happen.
"United are playing very well this season, they played fantastic against Real Madrid. But 10 games until the end is a difficult moment because all the teams can be tired after eight months. We hope."
The City boss also believes fixture congestion United may experience - particularly with an FA Cup quarter-final replay to come against Chelsea on April 1 - could be advantageous for his side.
"For this reason [fatigue] – because when you play one game every three days, it is always a top-quality game, there will be a moment that you are tired," Mancini added.
"I was not surprised [about the Chelsea result]. Sometimes they didn't play very well but won because they have good players. For a top team it is important to win when you don't play well.
"The next two or three games will be crucial. If they lose a point in the next two games, maybe [we can catch them]. After the two games, we have the derby [on 8 April]."