That's precisely the issue. I think if/when we sign a RB, we should go for a very good player/someone with the potential of becoming one of the best in the position.
And that will cost.
Bellerin and Cancelo been the names mentioned the most. But neither Arsenal or Valencia will let go of them right now. Unless Roberto/Vidal shows they'll cement themselves in the upcoming season, we've got to shop next summer.
Bellerin's contract runs out in 2019 and Cancelo's 2021. And there should obviously be other options too. But the most obvious one is Bellerin and we do love Arsenal players don't we? If he doesn't extend his contract then I'd say we got a good chance of getting him next year with two years left, whether that'd be a good move or not is another question, but not that many options out there either.
Roberto would obviously play where he's told to play, but he still remains adamant he is first and foremost a midfielder. Hopefully Aleix will fight and has sorted out his issues with Lucho. The former usually did well when he played at the RB position, but there's a different thing having to play there the entire season with all the pressure and demands of the position, contributing actively in attack and defence.