Roberto was left exposed because Chelsea outclassed Barca tactically. Barca were completely outnumbered in the centre of the park because Hazard and Willian went so narrow, it was basically Hazard-Giroud-Willian-Kante-Cesc in the middle vs Busquets-Rakitic and Iniesta. Roberto was very likely instructed to follow Hazard wherever he went. Unfortunately, Roberto is not a good enough defender to deal with a player with Hazard's dribbling and quality on the ball. Him following Hazard left oodles of space for Marcos Alonso to move into and attack. It's not as if on the other side things were totally different. Alba was busy with Moses and consequently Willian did as he pleased, waltzing through the Barca midfield. And at the same time, Giroud was pulling Umtiti out of position with quick one touch passing. Actually all 3 Chelsea forwards made a concerted attempt to play one touch and pull Barca players off position.
Yeah, Roberto had probably the worst defensive performance since the season started, and he's not a great 1v1 defender, but Chelsea's formation and ability to create numerical superiority left him completely exposed. Dembele had a great defensive effort to make that one tackle, but beyond that he really was not great defensively. It's not like he's a winger in a 4-3-3 who can chill upfront, a 4-4-2 while Chelsea are overwhelming Barca's midfield area demands more defensive work than he was able to provide or was interested in providing. Also I believe that Roberto was instructed to follow Hazard wherever he went, because Roberto does not end up to Umtiti's left normally in any game of his own accord.
It might be expecting too much of Roberto, a central midfielder, to be a tactically astute right back with elite defensive instincts, which is what was required yesterday.
He just cannot play against physical and faster teams and if EV wants to emphasize on defence then he isn't quite fit for a RB. Even when Dembele came off, Vidal was still doing most of the defensive work, Roberto just looks too vulnerable to fast wingers. Alonso and Willian were just dribbling past him like he weren't even there. I remember Semedo got blamed for a 2v1 situation against Valencia that resulted in a goal except Roberto was lucky not to have a goal conceded. Roberto was easily beaten in a 1v1, 2v1 and was way too short and slow to cause any physical presence for the wingers. Rakitic was another one who is also very slow.
That being said Umtiti and Pique needs to not both press too high up at the same time because at one point Busquets was left to mark Alonso and resulted in the alleged dive when Pique rushed back too late. They need to hold their positions and mark Giroud or Alonso, not both rushing up because of Willian. Understandably tho, they all were very afraid of Willians every movement.
I think in big games, Semedo still eventually needs to help out Roberto defensively. Both will have their strengths and EV needs to use them wisely. Roberto in midfield doesnt seem too bad either.
Willian was dribbling past pretty much every Barca player he came across yesterday. I may be misremembering, but it seemed like Roberto was actually the only player who managed to put in a successful tackle on Willian.
Edit: Actually I think Paulinho managed to put in a couple of tackles on Willian. Paulinho was not bad yesterday.