Some people are asking what position Marscherano plays.
He is a "number 5" in Argentinina football jargon, a DM who sits in front of the defense, to protect the back four and has a tidy distribution. I would compare him with Makelele or with Hamman but more dynamic.
I personally rate him much higher than Sissoko, who is the better athlete but far worse footballer.
He would be ideal in a 3 CMs set up, but I am not sure that's what Benitez is after, as he is a very strong advocate of Sacchi's 4-4-2 or 4-4-1-1.
Having Marcherano in the team would allow Xabi to play further up the pitch, but that would mean Gerrard on the wing unless Benitez changes his system (very unlikely).
When paired with Gerrard it would allow Steven to play a more attacking game as Marcherano is faster to cover central gaps, but I can't imagine Alonso sitting on the bench.
If Marcherano is signed Sissoko's minutes on the pitch would be reduced dramatically, but there may be problems with Gerrard having to play on the wing.