The match is finely balanced after United claimed a 1-1 draw in the first leg at the Bernabeu, meaning that Madrid must score if they are to have any chance of progressing.
Both sides are in sparkling form, with United unbeaten in 18 in all competitions and Madrid having beaten Barcelona twice in the space of a week, and Mourinho believes that the tie will capture the imagination of every football fan around the world.
"Who knows what will make the difference? That is the $1m question," said Ferguson.
"Tomorrow the world will stop for this tie. They are on a fantastic run, better than ever this season. FA Cup quarter-final, winning the Premier League in March, don't lose a match in months.
"We are also in good form in 2013. We have lost one match. It doesn't look a tie, it looks a final. Let's see which teams are at Wembley but I doubt the expectation can be bigger than this."
The match will also see Cristiano Ronaldo return to Old Trafford for the first time since his £80m move in 2009.