City had the best strikeforce last term - outscoring everyone with 93 goals - and the best defence, conceding 29 on their way to the title.
But following up that title-winning campaign was always going to be a tough ask and Mancini said: "It is difficult in any country to win back-to-back titles.
"Doing it in England, though, is even harder. We want to try to win it again but don't have the same quality this season that we showed in the first 15 games of last season.
"This season we have had big problems with injuries and the year after you win a title is always more difficult.
"We had a problem with the defence at the start of the season and conceded too many.
"Now we've improved that situation but we need to improve the output from our strikers. If we can start to score I think we can improve again."