The Reds go into the game at the Etihad three points ahead of their city neighbours at the top of the Barclays Premier League.
While the Blues remain unbeaten in the league so far this season, they have been held to draws in two of their last three games. Sir Alex says he and his players are well aware of the qualities Roberto Mancini's men possess and is urging his side to rectify recent defensive lapses in order to hopefully make it a memorable day for the red half of Manchester.
"It's going to be a massive game and if we defend like we did at Reading we'll be in trouble," the boss told MUTV after United's victory at Reading last weekend. "If we win [on Sunday] it will be one of our best ever results because they're a really good, powerful team with massive players. It won't be easy."
Commenting further on the derby in his exclusive United Review column for the Cluj match, he explained: "It's a big, big game for what, at least on paper, is a match between the two best teams in the country.
"As the table tells us, there is not much between us, though I recognise that City hold the crown as reigning champions. We need to come out unscathed and, with so much at stake, I wonder if there will be a little bit of caution from both clubs. Roberto Mancini and I will both have to think deeply about our selections and tactics and there may well be an element of playing safe.
"But don't count on it because it's a derby match when anything can happen and often does. The passion of the fans can easily spill over and affect the players so that tactics go out of the window as