Real Madrid's 4-4-2 formation worked really well against FC Barcelona last October. Kroos, Modric, Isco and James controlled the game during the whole second half, and Messi did not have a chance to get involved after making the play which created FC Barcelona's only goal.
But many things have changed since that last time these two teams met. Modric picked up a severe groin injury that kept him out of the field for 4 months, Toni Kroos has provided enough evidence to state that his legs are a bit tired, and so has Isco. Furthermore, James Rodríguez will not be available and he will be replaced with Gareth Bale. If Ancelotti is going to play with this same 4-4-2 formation -- something that should not be questioned considering how successful this tactic was against powerful and possession-oriented teams --, will that four-man midfield be enough to stop a much more focused Lionel Messi?
That will arguably be the key to this game. Kroos' lack of defensive instincts and strength could really hurt his squad, but it could all ultimately come down to how Ramos helps the German player. If Pepe and Modric take care of Suárez, Ramos would be able to take a step forward and prevent Messi's runs with the ball, especially when Barcelona try to counterattack. But then, it could all be reduced to a one-on-one matchup between Neymar and Dani Carvajal unless Gareth Bale takes his defensive commitment to another level.
As stated above, Real Madrid's success when Barcelona have the ball will rely on how supportive these attacking players are. If Isco and Bale -- the ones who will be on the wings -- stay focused and help Kroos, Modric, Carvajal and Marcelo, Real Madrid's midfield should be solid enough to contain Barcelona for a while, even without a pure defensive midfielder.
The whole world will be watching, so it will not be hard for Carlo Ancelotti to motivate his players and ask them to contribute defensively once and for all. That's what he needs if the Italian coach still refuses to play with a deep midfielder capable of helping the team's defensive line.