It's the matchup that Manchester United fans both feared and salivated at the prospect of only a few hours ago.
Jose Mourinho's too—let's not forget his illustrious history at the Theatre of Dreams. It seems like only yesterday when, as the then-Porto boss, he ran down the tunnel giddy having seen his team trump the European giants on their own turf.
Many will write off the Red Devils' chances—speak up the undeniable talents of CR7, Iker Casillas, Xabi Alonso, etc. But the reigning La Liga champions have not looked anywhere near the dominant force they were last season.
Sir Alex Ferguson's men, on the other hand, may not have played anywhere near their best this term, but have stepped up for the most important games on the fixture list.
Though it will certainly prove a daunting task, a full-strength United are certainly capable of beating the Spanish giants.
Here's a look at Real's strengths and weaknesses, and the keys to what will prove the most eagerly anticipated game of the season.
Under Jose Mourinho, Real Madrid have developed into a team equally adept at breaking down opposition defences both through the middle and from wide positions.
The side's full-backs, Marcelo and Arbeloa, like to push forward as much as possible down the flanks, often overlapping with the likes of Angel di Maria, Mesut Ozil and Cristiano Ronaldo in midfield.
In this sense, there is a similarity in their attacking philosophy with that of United's—that to truly stifle a team, pressure must be placed on them across the breadth of the pitch.
As seen in some of Real's recent matches, when the wings aren't fully utilised, the team looks rather two-dimensional going forward. Sir Alex must take heed from this.
It is no coincidence that Liverpool turned to custard not long after Xabi Alonso left for Spain. The holding midfielder is one of the very best footballers on the planet—and perhaps the best in his position.
According to WhoScored.com, the playmaker averages more than 70 passes a game—over a dozen more than any of his teammates. He is the spine of the Real midfield, the source of the great river.
Though Wayne Rooney and co. can have little hope of completely preventing him from operating, efforts must be made to deny him as much space as possible on the ball.
If not, memories of England's destruction at the hands of Andrea Pirlo in the European Championships may become a recurring nightmare for the homegrown players in red.
Real will inevitably have more possession than United in the Champions League clash; it's what Sir Alex's men do with the ball when they get it that's important.
The key in this respect is patience. The Spanish side often have a propensity to push most of the midfield forward, leaving gaping holes at the back should a team quickly break.
Manchester United have always been known as one of the most efficient counterattacking teams on the planet and will need to live up to that billing come February.
Wingers Antonio Valencia and Ashley Young will be key, as will Robin van Persie, who must play both the No. 10 as well as the No. 9 in swift attacking moves.
When Cristiano Ronaldo first arrived at Old Trafford in 2003, detractors thought of him as a speedy winger with a few stepovers in his locker and not much else.
Over time, he developed every facet of his game to the point that he became an ultimate player—devastating with the ball at his feet, powerful in the air and a true passer.
At Real, he is constantly relied on not only to score his team's goals, but to create them too. He is at his best playing through the middle, either making attacking runs into the penalty area or feeding others.
If the Red Devils can push him out wide, they stand a far better shot at containing his explosive presence. Of course there's always the increasingly creative Karim Benzema and the deft Mesut Ozil, but one problem at a time.
With all the ability of Real's galacticos, egos are always going to come to the boil in moments of frustration.
Central defender Pepe is a prime example of this. His volatility and occasional lack of self control is an issue that can hover over Los Blancos at times, like an impending storm cloud.
Jose Mourinho naturally thinks of himself as a master in managerial mind games, but like his star player Ronaldo, often lets himself down with some prima donna antics.
It may not be pretty, but led by the wily, old Scot Fergie and the brutish Nemanja Vidic, United must play the aggressor of the two clubs. It may just be a case of last team standing.
What do you think will be the keys to the UCL clash? Who needs to have a big game for United?