After being held to a 1-1 draw at the Bernabeu last month, Mourinho's side must score at Old Trafford to have any chance of progressing to the quarter-finals.
Mourinho insisted the last-16 tie "looks like a final", and he also claimed United had already wrapped up the Premier League title with two months of the season still remaining.
"Who knows what will make the difference? That is the $1million question," said Mourinho, whose side have lost only once this year and recently beat Spanish rivals Barcelona twice in the space of five days.
"The world will stop for this tie. United 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. But 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."
In the last 16 in 2004, Mourinho famously charged down the Old Trafford touchline after Costinha's last-minute goal sent FC Porto through to the quarter-finals, and they went on to be crowned champions that season.
"It means the same now," he added. "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."
Mourinho has confirmed goalkeeper Iker Casillas is not fit to okay after only returning to training on Thursday. The Spanish international sustained a broken hand against Valencia on January 23 and, although he travelled with the squad, he will not be considered.
"Everyone is fit and ready to play and be considered, with the exception of Iker Casillas," Mourinho said ahead of the second leg of the last-16 tie.
"We are sorry we had to travel yesterday (because of a baggage handlers strike) because we don't like to wait.
The match marks the first return of ex-United forward Cristiano Ronaldo to Old Trafford, and much of the pre-match focus has been on him with Sir Alex Ferguson having spoken in glowing terms about his former charge.
"I think he is fine, as far as I am aware," the Madrid boss added.
"I think any accolade he gets is more than deserved and I think the reception he will get from the crowd will be deserved.
"I am aware how well he did here and what an impact he had at the club."