The champions have struggled to replicate the intensity of last season's dramatic title run-in at the start of the defence of their Barclays Premier League crown. The pressure is now on as City, trailing by three points, battle with leaders United for top spot and local pride on Sunday but Mancini is far from worried.
City are still unbeaten in the Premier League and Mancini said: "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. 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."
He added: "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."
Mancini also feels the task of defending a title is much harder than winning it for the first time. The 48-year-old can speak with experience on that subject having won three successive Serie A crowns in charge of Inter Milan.
He said: "I think it is difficult in every country to win two titles in a row but to win here is very, very difficult. This is normal. This can happen to a young team like us that won its championship after 40 (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.
"We want to try because we are champions and we need to defend this title."
City are hoping playmaker David Silva can recover from a hamstring injury to feature after missing the midweek trip to Dortmund.