City pummelled United 6-1 in the Premier League earlier this season - a result that sent shockwaves across the English game at the time.
Now, Milner has warned that there could be a backlash from Sir Alex Ferguson's charges when the teams take the pitch in the FA Cup third round at the Etiahd Stadium.
"Beating them at Old Trafford in the manner we did will have hurt them badly," said the Englishman on City's official website.
"Everyone, from their players to the management and fans, will be more than up for it on Sunday.
"You'd imagine that as much enjoyment as we got from that win, they probably felt three times as much pain and we have got to be aware of that
."It is down to us to put on another good performance and build on the win over Liverpool. It was important to get that result after a couple of disappointing ones.
"United have won so many titles over the last 20 years and you don't do that by being weak mentally and letting a defeat put you off the rest of the season.
"We have got to be single-minded, that's what we have done all year.
City will go into the game at Eastlands as favourites, and are also leading United by three points at the top of the Barclays Premier League table. Milner however, is not taking anything for granted.
"Spurs are very strong, then there's Liverpool, Arsenal and Chelsea. There are a lot of good teams pushing to finish at the top of the league. We will just do what we have done all year and concentrate on our own football.
"If we can win our games, nothing anyone else matters, but we know this game will be anything but easy, especially with United coming into it on the back of defeats against Blackburn and Newcastle.
"We are the FA Cup holders and we want more of the same. This is a huge game and we want to progress in this competition and, if we can, retain the trophy."