On February 13 football fans around the globe will get to see two of the most well-supported and historic clubs in Europe.
Jose Mourinho billed it "the match the world wants to see" and said, "Its only shame is that it's not the final." (per the Daily Mail)
We will see two of the great coaches in history try to outfox the other for a place in the final eight of the tournament.
Clash of Titans
Real Madrid may have the slight advantage in individual talent, but Manchester United has been the more cohesive team this season.
Almost every matchup on the pitch can swing the game one way or the other. There are plenty of playmakers in this tie that can turn a match on its head in a moment.
Guys like Cristiano Ronaldo and Robin van Persie have a reputation for single-handedly winning matches, but other top stars like Mesut Ozil and Wayne Rooney are equally capable.
While no single player will be solely responsible for beating either of these great teams, there is one person who will be the key for Los Merengues if they are to stop the nearly unrivaled attack of the English side.
That person is Xabi Alonso.
Alonso the Key to Victory
At full health and in top form, most objective fans would admit that Real Madrid are a better team than Manchester United.
Unfortunately for Los Merengues, they have not been even close to top form all season and will enter their match against the Red Devils very short of full health.
The reigning Spanish champions are finally starting to hit their stride, but key injuries to the defense will put them at a huge disadvantage against one of the most in-form finishers in football.
Because of this, the man protecting the back line will be pivotal. This means that Xabi Alonso (as well as Sami Khedira) have to be in top form.
Madrid's Central Defense in Trouble
At their best, Sergio Ramos, Pepe and Iker Casillas would struggle to stop Robin van Persie and Wayne Rooney from scoring, as would any team in the world.
Jose Mourinho will not even have the advantage of playing that world-class trio when Sir Alex Ferguson rolls into town.
Now, having Diego Lopez between the posts will not be the same as having Casillas, but anyone who thinks Lopez is not a top-class keeper just hasn't watched him in recent years.
The central defense, on the other hand, will be well short of their world-class best.
Sergio Ramos has been excellent, but inconsistent this season. Pepe has been even further from his best, but he would still provide a huge boost at the back.
Unfortunately, Pepe will probably miss out with an ankle injury, which means that Ramos will have much more work to do.
Ramos' defensive partner will probably be Raphael Varane then. While the potential is enormous, Varane is still very raw and relies more on his physical strengths than his knowledge of the game.
Because of the weakness here, Xabi Alonso will have to be at his best to protect his center-back pairing and make life easier for the 19-year-old.
Alonso will he be overloaded in midfield, just limiting chances created and cutting off passes, but he will have to do a lot more than that.
Jose Mourinho could decide to go with Raul Albiol instead of the young Varane, but in all honesty, that would only further weaken the defense and make them even more vulnerable on set pieces.
With Varane in, Alonso will have to play a deeper, more defensive role to assist the Frenchman.
He'll have to double team whichever striker moves into the youngster's zone and will likely be the last man at times when the central defense is stretched.
As great as Varane is in the air and as incredibly talented as he is more his age, he still has trouble with his positioning, seeing lanes and staying in his zone.
Just as we saw against Osasuna and Real Sociedad (among others), Alonso makes the teenager look a bit more poised and in control than he is at times.
End to End
It is vital to Madrid's success that Alonso take pressure off of the central defense. However, his duties are just as valuable at the other end of the pitch.
Los Merengues are one of the absolute best counter-attacking teams on the planet.
While the pace and finishing of Cristiano Ronaldo and the vision of Mesut Ozil are the reason goals are scored, everything starts from the back.
Most often, it is Alonso who provides that first pass and starts the Madrid attack.
His ability to intercept a pass, realize where he and his teammates are and then send the ball into a hole for his transitional players is almost peerless.
Madrid's No. 14 will need to be at his elite best if his team is going to break through United back half and give his forwards a chance to score.
Alonso the Defender and the Playmaker
Not only will the former Liverpool man have to start the transition, he will often find himself in position to provide that final pass for Ronaldo and the strikers.
Alonso's vision and long ball passing are among the best in the world, but he will have to be very smart in deciding when and how far to venture forward.
If he can pick the perfect time to send that killer pass over the defense or even spread the attack without exposing his defense, Madrid will have a great chance of limiting the damage at home.
Most of what Alonso must do will go unnoticed and unappreciated by many even if he executes perfectly.
As long as his team wins, I don't think the lack of recognition will bother the great Spaniard too much.
Away goals could be the deciding factor in this tie so preventing the Red Devil attack from scoring more than a goal at the Santiago Bernabeu is hugely important.
Madrid are likely to net a couple of goals over both legs as well, but getting in a shootout with RVP while the defense is short of full strength would be risky.
Jose Mourinho is a master of two-legged ties and has had enormous success in the Champions League so his tactics will likely be sound.
However, the preparation from the manager is only part of the battle. His players' execution of strategy is the other piece of the puzzle.
While the entire team needs to be focused and not make mistakes, Xabi Alonso's role in this tie is the single most important key to victory for Real Madrid.