I agree Mascherano is a great player, but will rafa be willing to part with 17m next year to keep him. I just think the reason he's kept everyone who play's CM is that if Momo improves his passing, and Lucas settles, Alonso and Gerrard will have 4 years left on their contracts he may not feel the need to buy him.
Of course this is all way off yet, but I think everyone will get a chance to prove themselves in midfield, and with Mascherano not being a definate for the future, why put his name on the team sheat first, when we have so much talent signed up.
We all know how Rafa likes to keep his options open.