City beat Arsenal 2-0 at Emirates Stadium on Sunday - their first win away to the Gunners since 1975 - to stay seven points adrift of United, who had seen off Liverpool 2-1 earlier in the day.
Mancini admitted United are currently in good form but remains adamant the reigning champions can close the gap on their local rivals and retain their title.
"United are a really strong team," he said. "In this moment, they play very well. But the season is long. It's important for us to be there, close with them, and we will arrive the moment they lose points.
"We're under pressure maybe before, but seven, eight or nine points, everything can change at the moment. We have the derby on April 6 and we need to arrive there with five or four points behind, it is okay."