City crushed their neighbours 6-1 win at Old Trafford last season and secured the spoils again on home turf in April with two games left to steer their way to a first top-flight crown in 44 years.
With second-placed City breathing down the necks of leaders United, the season enters a vital period before Christmas and Mancini said: "There are some important games that can change your season.
"The derby last season was one of the first games like this for us. It was important for us because we played the first derby away and we won 6-1.
"They have three points more than us and they have a small advantage but a derby is a really difficult match for both teams, and it is important."
The champions have struggled to replicate the intensity of last season's dramatic title run-in at the start of the defence of their Premier League crown.
Having also failed to make the knock-out stages of the Champions League, the pressure is on but Mancini remains unfazed with the added pressure to perform on the domestic front.
"I think it is difficult in every country to win two titles in a row but to win here is very, very difficult," the Italian added.
"This is normal. This can happen to a young team like us that won its championship after 44 years. The day after is more difficult because you need to work more.
"The first thing is to forget everything you did until here and work more because it is normal that it is more difficult the second year.
"But with all the problems we had from the start of the season, to be there, only three points behind United, is a good thing in this moment.
"Against Chelsea we dominated the game and were missing only a goal. Against Arsenal we deserved to win and they scored only in the last 10 minutes.
"I don't think we have the same qualities we had last season in the first 15 games but I think this year we had big problems with players injured."