Well that's the thing, Abidal is not the coach though. Abidal may see Roberto as a Midfielder but Valverde sees him as a RB first and foremost.
With the likes of Rakitic,Busi,Coutinho,Paulinho being around for Midfield options. And then you have Arthur coming in and Alena being promoted and when he gets fit and you also have to take into an account if we also buy another CM like Pjanic/Thiago etc. Heck Valverde likes Oriol as well.
It will be insanely difficult for Roberto at this point to ever go back to being a full time CM again. He already has the established Midfielders in front of him and the potential ones in Arthur and Alena and even B team options like Oriol as well.
I feel like one of the biggest reasons why Valverde used Roberto as a RM in the CL matches is because we didn't have Coutinho available to play in the CL as he was cup tied and Paulinho was a disaster as a RM as well.
Roberto has flaws at RB but right now that's his only real chance of playing and contributing the most for the team.