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City travel to Old Trafford trailing United by 15 points at the top of the Barclays Premier League and, with just eight games remaining, Mancini has conceded defeat in the title race.
But he is convinced the sizeable gap is not a true reflection of the difference between the sides and he wants City to show that on the field.
The Italian said: "To play there is difficult - the stadium is big, their supporters, their staff is strong.
"But there is not a big difference between us and them.
"If it is not important for the table or the title race, I think the derby is always a derby. It is important for us and for them.
"We want to show we don't deserve to stay 15 points behind and we can reduce the gap from now to the end.
"But the title race is finished."
With the FA Cup now City's only realistic hope of silverware, it would be understandable if priority was given to next week's semi-final against Cheap chelsea shirt at Wembley.
But that is clearly not City's mentality, with Mancini's words backed up by those of a number of players this week.
After so many years of living in United's shadow prior to their 2008 takeover, City are established to make a statement to the neighbours.
Striker Edin Dzeko told the club's website, www.mcfc.co.uk: "It's a very important game even if we are 15 points behind because the derby is always the derby.
"It's the best league game of the season and we will do our best to try and win there.
"To win the derby always means a lot - for us and for our fans it is very important."
Dzeko scored twice when City made one of the most emphatic statements of their recently-elevated status among the country's elite by thrashing United 6-1 in October 2011.
That was City's last visit to Old Trafford and the match will long be remembered in the blue half of the city but nobody is seriously expecting a repeat this time.
"This is impossible," Mancini said. "This can happen every 100 years."
City are close to a clean bill of health for the game with only right-backs Micah Richards and Maicon, who have both been sidelined with knee injuries, definitely ruled out of contention by Mancini.
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