It is ludicrous how oblivious you are to the fact that Sergi Roberto does not have a similar playstyle to Xavi, Busquets, Thiago or Iniesta. Sergi Roberto is a player suited to the wide areas. He does not have the intelligence nor the awareness nor the ultratechnical skill needed to operate closer to the centre of the park in a team of Barca's ilk. Moreover, he is very intelligent in the wide areas: in fact he is seriously smart in the wide areas. He knows how to contract or widen the pitch. He is suited to the wider areas, his playstyle and intelligence orientation is more like Lahm. He would be an absolute failure in the centre.
Xavi, Busquets, Thiago, Iniesta. Funny how you left out Rakitic there, because that's the role everyone expects to see him play at, not an Iniesta or Busquets role. If you're telling me Rakitic has some superior intelligence or awareness over Roberto, well then
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Roberto as a RM will be fantastic. He can link-up on the wide areas with the RB, have dummy overlaps with the RB taking up the RM spot when he does go up the wings. We've seen it before when Alves used to do that as well.
Also, you don't seem to have seen any of his trademark runs, most of which are right through the middle of the pitch; something we won't be able to see much of if he's kept out wide.