It is a real conundrum for Rafa as to how to play his central midfielders and a task i don't envy him. The fact is that individually Xabi, Momo and Gerrard are all very different players who work in different ways together.
On paper the "best" two players would be Xabi and Gerrard but then you lose the bite and defensive qualities of Momo, essential ingredients in any successful premiership team. OK so you have to play Momo but who with? Do you drop either of your two most influential players?
The answer that Rafa has obviously come up with is "No" and so you play them both. Xabi and Momo work best together in the middle, that has been proven and also Xabi is less versatile and couldn't play wide like Gerrard and so you are left wirth Gerrard out on the wing.
What else can you do? Change the formation, play Gerrard behind a lone striker? Put three in the middle? No, the team works as a 4-4-2 and so it shall remain.
For me Gerrard is the best central midfielder in the world, including Xabi. I don't think that Xabi is a much better passer and I certainly don't think he tackles and defends better than Gerrard, Gerrard is a more complete midfielder without doubt BUT in the context of our squad and formation and the tactics employed by our esteemed manager the only place for Gerrard is on the right side.
Short of buying new players to fit around him it isn't going to change unless injuries dictate otherwise, I just hope that Gerrard is happy with this as we need him focused and playing his best if we are get back up to the top of the league where we belong.
This is my post from another thread but it seems relevant here too. To summarise: Do we need Steven Gerrard? Very much YES! Do we need Gerrard in central midfield? No