There's a really good quote from Franco Baresi regarding the signings of central midfield players for Real Madrid this season. I'll try and find it...
“One thing does not fit,” says Baresi. “Why have they [Real Madrid] hired Xabi Alonso? Xabi Alonso is a good player but he serves the same profile as Gago, Granero and Mahmadou Diarra. They had to sign Felipe Melo. He is technically and physically superior and is able to give a new dimension to the midfield. Lass, Xabi and Granero play on the same plane. It is a mistake to put them together. The only one who breaks the line is Guti.”
I think that Alonso is better technically than Melo but the point he's making is very good. Liverpool realised this season that Alonso was truely crucial because he was the link between Mascherano and Gerrard and offered a different dimension. If you have a flat line of players, there's a lot of predictability in your attack. De Rossi will offer something different from Alonso and Madrid should be a lot more fluid in transition.