Personally, I don't see the worry here, you must have been brainwashed by the Italian mentalists.
Gago would be more of the holder, play a bit deeper to pick up the ball from the back. Marchisio would drive forward with the ball, but he'd also have the awareness to stay back when the pressure is on, and he can defend at a reasonable proficiency.
It would be a little bit like Carrick and Scholes.
We don't need a destroyer in there. None of the top clubs in the world have a one-dimensional player holding such an important role to the side, the defensive midfielder. And in fact, the ONLY bright spots we saw with regards to possession last season came when we had neither Sissoko or Melo in the team, instead relying on Candreva, Diego and Marchisio. It's not a coincidence either -- they simply could play the ball, Sissoko and Melo could not.
It is a very simple matter.
Juventus, and Juventuz, have become conditioned to think that a destroyer must be fielded no matter what. That is simply nonsense.