Mancini has 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."