United have seen an eight-point advantage slip away thanks to a defeat at Wigan and last weekend's 4-4 draw with Everton.
A victory will move City level on points with the champions with better goal difference and just two matches remaining.
But while City head to Champions League-chasing Newcastle and then host relegation-threatened QPR, United face matches against Swansea and Sunderland that Mancini has dismissed as "easy."
City had led the table for five months before their own winless three-match run and Mancini feels that whatever happens on Monday, United should still win a 20th title.
Mancini said: "I think they have more chance than us because we play derby Monday after we play against Newcastle, that will be a very tough game.
"Instead United play Swansea and Sunderland, for them, they will be two easy games. If we win this game, they are favourites.
"I have a big respect for United because for me they are the best team here and maybe in Europe with Barca, Real Madrid.
"(When we were ahead) I used to wait always until the end of the championship because in football we lose seven points in four games, this can happen for the other team. You can lose a point everywhere."
City are hoping for a first league title since 1968 and despite leading for so long, Mancini maintains he will be happy with the club's progress even if they fail to win the league.
Mancini, who has a year remaining on his contract, saw his team's first ever Champions League campaign end in the group stages but feels the squad have made progress.
The Italian added: "I think we should be proud for our season because we did 80 points, we won the last three games, we have another three games. Also if we beat them it will be very difficult for us.
"In the last two years we improve a lot and this is very important. It is important we are here at the end of the season we play for the title."
United have prepared for the game with a training break in Wales and although Mancini insists his own team's preparations have not changed, he joked that he hopes Sir Alex Ferguson's squad remain in Cardiff.
"If they want to stay there until Tuesday! If they don't come we win the game!" he said.
Mancini, who has a fully-fit squad for the United match, has hinted he will consider using Mario Balotelli after the striker returned from his third suspension of the season.
Defender Micah Richards had been doubtful with a hamstring problem but is fit to play.
The manager claimed Balotelli would not play for the club again following his red card in the 1-0 defeat at Arsenal earlier this month.
But Balotelli is now back with the squad and could feature against United but Mancini has yet to make up his mind whether the controversial striker, who starred in the 6-1 win at Old Trafford in October, will start the match.
After helping the club to three consecutive victories, former United striker Carlos Tevez is expected to start in attack alongside fellow Argentina international Sergio Aguero.
Mancini said: "All the players are available. We have another two training sessions but we have all the players available. I have 48 hours to decide this."
Source: AFP
Source: AFP