Real Madrid coach Jose Mourinho believes the world will stop on Tuesday to discover the answer to a 1million dollar question at Old Trafford.
Mourinho has brought his Madrid side to Manchester knowing they must score against United to stand any chance of preserving their Champions League place.
It is a clash of the titans, which had even put AC Milan versus Barcelona in the shade until the Italians recorded their stunning first-leg win last month.
Nevertheless, interest worldwide is huge and Mourinho knows he is centre stage.
"Who knows what will make the difference?" he said. "That is the 1million question.
"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."
Nine years ago, at exactly the same stage of the competition, Mourinho came to Old Trafford for a second-leg tie and ended up charging down the touchline in delight at a last-minute Costinha goal that sent FC Porto through.
Since then, one of the most charismatic figures in the game has won domestic titles in four different countries and is currently aiming to become the first man to coach three teams to Champions League glory, following success that season with Porto before lifting the trophy with Inter Milan in 2010.
Yet Mourinho insists nothing much has changed since that day in 2004 when he announced himself to the English game.
"It means the same," he said.
"I look at the situation with different emotions and the approach is completely different.
"That match was my first season in the Champions League. Now I have more than 100 matches and I can control my emotions in a different way.
"If I lose tomorrow I don't cry. If I win, I don't run 100 metres.
"But the happiness and sadness will be the same; whether I am sleeping well or not, whether I laugh a lot on the plane or pretend I am asleep.
"That is the same story."