He has not played better than Semedo.
He
must not play somewhere. He's Sergi Roberto, not Lionel Messi.
But if he must play somewhere, it should be in Rakitic's position.
As for your question @
Origami
While I want him to get a run in the midfield, I don't think he'll be a starter for Barca long-term. Eventually better midfielders will take his place simply because they are more talented and that actually is a fact. Roberto has transformed into a good player who got some nice attributes such as his bursting runs and stamina, but you only need to compare him to a former La Masia graduate he played with to notice the difference in quality. You know who I am talking about. In fact in his age group you have other players such as Koke and Isco too.
Roberto is a part of the supporting cast. He's not going to be a lead star which Barcelona have to revert back to, find a true WC midfielder or two.