Having Lucas/Mascherano/Aquilani(/Gerrard) in any combination depending on the opposition is a good thing and we should keep all of them. Replacing any of them with another, possibly lesser, player and investing in other areas of the pitch is too premature. They're just now getting a chance to play together.
But Lucas/Aquilani against Chelsea's, Barcelona's or Real Madrid's midfields wouldn't cut it I believe. We need Masch for those games.