It has been an amazing season for the Blues so far, capped by that staggering 6-1 away win at Old Trafford in October.
They reached the end of the festive programme three points clear of United and well placed to end the season as champions for the first time since 1968.
However, as Sir Alex Ferguson brings his players to the blue half of a divided city for an eagerly-anticipated FA Cup third-round tie, Mancini acknowledges what a boost lifting the trophy last season gave everyone at the Etihad Stadium.
United were accounted for in a Wembley semi-final before City returned to what is dubbed the 'Venue of Legends' by the marketing gurus to defeat Stoke and end a trophy drought that extended to 35 years.
"It was really important for the club," said Mancini.
"When you build a new team you have to start to win very quickly.
"If you get one cup, in that moment you can understand if you work hard and win the FA Cup, you can win the championship."
Unfortunately for Mancini, the inspiration behind those wins against United and Stoke is not available this weekend.
Yaya Toure has been refused permission to delay his departure for African Nations Cup duty with the Ivory Coast, so will be heading out to Abu Dhabi with his international team-mates as his club colleagues prepare to do battle.
It makes the controversial dismissal of Gareth Barry against Liverpool on Tuesday all the more significant, although if there is a consolation for Mancini, it comes from the fact no league points will be at stake this weekend.
Indeed, Mancini is dubious about the effect this weekend's result will have on the Premier League scrap, beyond the desire to retain the trophy.
"The FA Cup is important for us," he said.
"We won it last year and want to go in the final again if it is possible.
"It is a difficult game. It is a derby.
"It is harder because they are angry after two defeats and they are usually very strong after a defeat.
"But it is not important for the Premier League, other than maybe physiologically."
Mancini concedes United's thirst for success could hardly be greater given the recent defeats and that huge one at Old Trafford.
However, it is on days such as this City need to grow into the club they hope to become.
And that means sizeable contributions from all their players, including Edin Dzeko, who has now gone 10 games since his last goal after netting 13 times in his first 16 matches this season.
The Bosnian drew another blank against Liverpool, when he was overshadowed by strike partner Sergio Aguero yet completed the full 90 minutes when the South American was replaced 18 minutes from time.
It seemed a strange move.
However, Mancini is happy with the man he spent £27million on 12 months ago.
"I am happy with Edin," he said.
"He didn't score against Liverpool but he was really strong.
"He played and worked for the team. This is very important for me.
"I am sure he will start to score very quickly."