The two clash on Sunday at the Etihad Stadium with United currently holding a three-point lead on the champions.
Even though the season is not yet halfway through, Rafael knows the meeting with their fierce rivals is a great chance for United to open up a significant lead.
"If we open the gap it can make the other team nervous and make it easier for us," said the Brazilian.
"There is definitely a determination to get the title back after the way we lost it. It was hard to take.
"We will never forget how we felt after the last game. We just looked at each other and asked, 'How did we lose that?' After last season, we just want the title back."
Rafael was taken off in United's last game at Reading after just half-an-hour as they struggled to cope against the Royals, before edging a seven-goal thriller.
And he admits that they were not good enough despite their 4-3 victory at the Madejski, adding: "If we defend like we did at Reading against a team like City we will be in trouble, that is 100 per cent.
"This will be our biggest test of the season so we need to concentrate well."