With the 4 CMs being Marchisio, Melo, Sissoko and Aquilani (?), I don't think that a single one of them has a guaranteed starting spot. Even without buying the Italian, Marchisio, Melo and Sissoko don't have a guaranteed starting place.
From now on it will be
1. Who plays better.
2. Which pair develops better understanding and gives better balance.