After falling 12 points behind leaders Manchester United in their bid to retain their Premier League crown, the knockout competition appears to represent City's most realistic hope of silverware.
Mancini may have rubbished suggestions he could face the sack if beaten by the Championship outfit in their fifth-round tie, but he remains aware of the game's importance.
After starting the season with such high hopes in the Premier League and Champions League, he is anxious to ensure the campaign does not disintegrate.
The Italian said: "We play with the best team. I will do some changes because I have two or three players injured but after that, I will play with the best team."
Although captain Vincent Kompany will not be risked as he continues his recovery from a calf strain, the only deviation in that policy will see back-up goalkeeper Costel Pantilimon given a chance.
Mancini said: "I will play Costel like always in the FA Cup.
"Costel is a good keeper, he deserves to play. He always does in the FA Cup and he did well against Stoke."
First-choice Joe Hart will therefore be rested but Mancini stressed that has nothing to do with the much-highlighted error the England goalkeeper made in last week's damaging loss at Southampton.
He said: "For a keeper it's normal sometimes to make a mistake.
"If the keeper makes a mistake you lose a game but if the striker doesn't score a goal, maybe you can draw 0-0.
"Their job is difficult because of this."
Mancini was angry and heavily critical of his players after the dismal 3-1 defeat at St Mary's Stadium, which gave United the opportunity to tighten their control of the title race.
The City boss expects much more from his side and remains at a loss to explain the below-par display against a team battling against relegation.
He said: "I said I was disappointed with them because they played a bad game.
"I don't think a team like us can play a game like this.
"We can lose when the team we play perform better than us, but not like this because a team like us should be trying to score, score, score.
"We did very, very badly. We didn't play as a team, we didn't fight for every ball.
"Sometimes it is impossible to win all the games because we are a better team.
"Sometimes it is important to fight because we can win 1-0 - not because we played better.
"It is impossible to play all of the games better than your opponents."
Standing between City and a place in the quarter-finals is a Leeds side hoping to push themselves into play-off contention in the Championship.
They claimed the scalp of Premier League Tottenham in the last round but they have not won since and, after an inconsistent season, manager Neil Warnock is under pressure.
Despite that, Warnock's reputation in the game remains good and Mancini remembers how his old QPR side gave City a scare when they met at Loftus Road last season.
Mancini said: "Warnock is a top manager, he has big experience.
"I played against him last year with QPR, we had big problems when we won 3-2 in London.
"He knows football very well."
Midfielder Gareth Barry is doubtful for the clash but defender Matija Nastasic if fit after further knee problems.
Striker Carlos Tevez, captain when City won the cup two years ago, is also available after returning from compassionate leave.
Tevez made a strong start to the campaign but has scored just once in his last 11 appearances.
Mancini said: "I hope he can score important goals in the last three months.
"The problem we had until now is we have scored less goals than last year. This is the gap at the moment."