of course he's going to struggle if there isn't anyone there to help him out.
In attacking sense, nobody is helping him because (at least Messi) have gave up on him more or less.
With Messi and some players you can easily see how they are always passing to a certain players and how they have a lot of faith in some players.
On the other hand, you can also see how a certain players never pass to some players, because they think that absolutely nothing will happen when you pass a ball to them, so a player1 is just looking for some other, better options.
Roberto at a RB is one of those players.
A player to whom you will rarely pass the ball, since in 9 out of 10 cases, absolutely nothing will happen.
He will just pass the ball back to you, or to a Cb or Busi.
Or he will try some dribble or cross, after which, again, nothing will happen.
** It slightly reminds me on a situation from a summer of 2015 (if I remember correctly), when Halilovic played for us in Usa during a preseason.
And whenever he came in in the last 10-20 minutes, he was always "free" on the right flank, no one was covering him but our players (except Rakitic) just didn't want to pass the ball to him and they just always passed to "someone else".
So, a guy was there alone and free but nobody gave a fuck about him.
Anyway, about Roberto, I wouldn't blame his teammates for not helping him.
i would blame Roberto for being extremely meh in attack to that level so that the whole right side is dead and majority of his teammates tried to play some other options with some other teammates (Messi as a No1).
Like (Messi thinking): there is absolutely no one to link up here (on the right flank). I'll just go to the middle, maybe I'll found someone to link-up there :/
Messi has to link-up with the weakest player in our team...