United have suffered two successive league defeats in the last week, Mancini said: "If you want my opinion, United are the best team in England.
"They have a strong team with a strong mentality and they are at the top."
But this has become much more than an FA Cup tie for United- not just because they have lost their last two league games but because of the financial implications of defeat.
Already out of the Champions League, chasing City for the title but not sure to get there, and three succesive defeats would send out alarm bells to the Glaziers and fans alike.
United's projected income has already been dented by the loss of Champions League football. And they face a double whammy as they may have to compound the clubs debts by buying players at a time when they usually sit on their hands.
Sir Alex Ferguson has only done a handful of deals of any note in the January window in the last five or six seasons. But the loss of Vidic and Fletcher for the season has exposed the pasuity of the United squad. Defeat today would result in mounting pressure for the United boss to get out the cheque book.
United are alreday being linked with Frank Lampard, Gary Cahill and Wesley Sneijder.
The worry for United fans will be if the owners decide the best way to strengthen the squad is to sell first to get some cash - Wayne Rooney is the obvious asset to dispose of. That's why United need a win far more than City.