It can be done either way. You can have a pure defensive player sitting back with two players in front of him doing more of the playmaking duties, or a deep-lying playmaker sitting back and a workhorse next to him providing the legs.
Personally I do prefer the latter, but more importantly it does suit our current players better. We'd only need to buy the one player, as we already have Herrera who is well suited to the workhorse role. It's also the same set-up that Pogba had at Juventus. The other way we'd probably have to buy two players now, as Pereira likely isn't going to make it and Gomes isn't ready yet.
Of course, the ideal would probably be having all three of them being fairly good at controlling the team around (similar to how Pep normally sets up).