By FOX Sports Staff
The Manchester City manager has a shrewd reason for this allegiance - he hopes that United will be worn out by their exertions and drop points later in the month.
United secured a 1-0 win over Fulham on Saturday and currently have a 10-point lead over their city rivals who face Liverpool on Sunday knowing that many pundits have already ruled them out of the title race.
Mancini's men faced Madrid in the group stages and lost to them at the Santiago Bernabeu in their first fixture despite taking the lead, with the result setting the tone for a dismal European campaign.
Having faced the Spanish giants, he knows that United will have a tough time achieving progress to the quarter-finals.
"Of course I will be supporting United when they take on Real Madrid. It would be another two games for them. I might even go along to watch with a little red and white flag," he told the Guardian.
When we played Real Madrid it took a lot out of us and I think United are going to find it difficult as well, because along with Barcelona [Real] are the best in the world right now."
"They have incredible players, fantastic history. It will be very hard for United to beat them because they are a really top club."
The Italian also dismissed concerns that City are too far behind and reminded those concerned with their feat in the previous season when they closed down a similar gap in the second half of the season.
Mancini said: "If we could do it last year when we were eight points behind with six games to go, it's not impossible that we can recover this with 14 games left.
"We could recover five or six points and still lose at the end but we'll do everything to win this championship. United have good experience but anything can happen right up until the last game.
"And we have a good memory of what happened last year.
"When you are seven points behind, it's difficult to recover but we're a good team and playing well. We've also started to work better with the defenders."