The Italian admits they have underachieved in Europe, but believes there is still much to play for this season.
City crashed out of Europe altogether on Tuesday night after finishing bottom of Group D with just three points which was the worst return of an English side in Champions League history.
City's poor campaign in Europe has increased the scrutiny on Mancini, but the Italian insists their main target was to retain the Premier League title this term.
"It's clear the season isn't finished. We have the Premier League and FA Cup to go for," Mancini told The Sun.
"But we must work more now. What we're doing is not enough.
"Our target when we started the season was to defend our title. That was the main target.
"We knew it was difficult to win the Champions League. But our target in Europe was to get into the second stage.
Mancini added: "Our targets have not changed. Our main target was the Premier League and FA Cup - to win a trophy.
"We wanted to get to the Champions League second stage - then in February. when you're there, you can change your target. But we didn't and now we can't.
"I don't feel any pressure. I'm disappointed because we went out of the Champions League and I'm disappointed for the club because they deserve to go through and for our players, who deserve to play on that stage."