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.
They did beat Tottenham last month but in other clashes against the competition's leading lights this term - Chelsea, Arsenal and Everton - they have been forced to settle for draws.
Their Champions League campaign was a dismal failure with losses to Real Madrid, Ajax and Borussia Dortmund seeing them knocked out at the group stage.
The pressure is now on again 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.
"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 appeared vulnerable in the early part of the season and did not keep a clean sheet until their seventh Premier League game.
They have improved in that area and now boast the best defensive record in the competition but Mancini feels problems have switched the other end of the pitch.
After 15 Premier League games last season, City had scored 49 goals. This time they have scored 21 fewer.
Mancini has called on his much-vaunted strike quartet of Sergio Aguero, Carlos Tevez, Edin Dzeko and Mario Balotelli to deliver more goals several times in recent weeks.
He said: "We had a problem with the defenders and conceded more goals at the start of the season.
"Now we improved that situation and we need to work to improve our situation with the strikers.
"I think that our season depends on our strikers. If we start to score like last year we have a big chance."
While losing and falling six points behind United would hardly mark the end of City's challenge with much of the season still to play, Mancini is well aware of the importance of derbies.
City's stunning 6-1 win at Old Trafford last season ultimately proved crucial in the final count as the title was settled on goal difference.
Victory over United again in April then swung the race back in City's favour with just two games of the campaign remaining.
Mancini said: "It was important for us because we played the first derby away and we won 6-1.
"There are some important games that can change your season and the derby was one of the first games like this for us.
"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."
City are hoping playmaker David Silva can recover from a hamstring injury to feature after missing the midweek trip to Dortmund.
Left-back Gael Clichy and midfielder James Milner are also hoping to return after ankle and hamstring problems respectively.