I'm in a very good position to talk about Marquinho.
He played 3 or 4 seasons for my favourite club Fluminense here in Brazil just before joining Roma.
And I have to say: the guy is mediocre. By mediocre I mean not that good, not that bad. Just plain average.
Great work rate; he runs like hell, chases opponents all the time, is tactically obedient, has lots of grinta, a decent left foot. Suitable for counters.
He is actually pretty similar to Giaccherini, but he is left footed. He can fit very well as an wlb, as Asamoah's substitute. He also can play in Marchisio's position.
Like Giaccherini, he leaves it all on the pitch. And like Giaccherini, he tries very hard, makes a lot of mistakes, and does stupid things sometimes. He's not very skilled or intelligent. But he's quick, he's got stamina, decent left foot, he can be very aggressive, he won't mind the bench. But as a 27 yr old, he won't improve much.
For 3/4M he could be useful, but just meh. His celling is pretty low. I'd like to see young hungry talented players being signed from abroad rather than seeing Conte settling for Italian mediocrity.
But that's the small price we have to pay for a great great manager. So be it.
Just PLEASE don't pay more than 4M for Marquinho.
Mark my words. That's EXACTLY the kind of player he is. I've seen him play 2x a week for 3 years here. He will never be a starting player for Juve or even for Roma. But he can be useful. Just hope it's the right price.