City cross town to Old Trafford 15 points behind Manchester United and on the verge of relinquishing the title to their bitter rivals.
Mancini has long since written off the champions' chances of a second straight success, but he is adamant the gap at the top does not accurately reflect the difference between the teams.
"It's not a true table," said the Italian, who did concede United will be worthy title-winners. "We have eight games left, and if we play well maybe we can reduce this gap. That should be our target.
"We'll (be motivated) because it's a derby and because we want to show maybe that we don't deserve to stay 15 points behind.
"It's not important for the title race, but the derby is always the derby. It's important for us and for them, and there is always pressure. Every derby is important.
"We want to go there and try to win. We know it's not easy to win there, but we want to play a good game, to do our job and we'll see what happens."
Asked to pinpoint the difference between the teams, Mancini identified a goals tally well down on last season's and warned that a failure to spend this summer could mean stagnation for City.
He said: "We've scored 20 goals less than last year - only this is the difference. The goals we conceded are the same, and we have the best defence.
"We're missing only these goals, but we had a lot of injuries to Sergio Aguero and we lost Mario Balotelli, who scored 15 goals last season.
"We want to improve our team (this summer), not like last year. We've only challenged for the title for two years, and if every year you don't improve it's difficult."