The Italian saw his side finish bottom of their Champions League group and miss out on a place in the Europa League on Tuesday.
He believes his team need to win the game as they are three points behind their arch rivals when they meet at the Etihad Stadium.
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."
The Italian feels the fact that Sir Alex Ferguson’s United side have been successful over the last two decades is something that will help the Old Trafford outfit.
Mancini added: "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."