It wasnt simply a fullback issue, his formation and tactics as a whole was simply counterproductive. He used a 4231, but with Paulinho rushing up to make a variant, 4141 and Messi is used to playing as a false 9 and dropped too deep resulting in 4-5 men upfront and Messi at the back. In this case, Messi playing heavy possession doesnt help, the purpose of 4231 is to give many attacking and passing options but Messi in this case tried to do everything. If EV is aware Messi is accustomed to a false 9, then he shud switch it to 433. Something like the game against Espanyol at home, look how quickly we scored as compared to this game and even conceded.
Too much vulnerability at the back with a 4231 and he even expect an ultra offensive fullback style; it will only leave Umtiti, Rakitic and Pique stranded. Not to mention he played Coutinho in the right flank.
Even Man Utd in 2007/08 played 4231 but played quick direct passing and fullbacks usually go for long passes because they are alone at the back but in our case Messi dropped ultra deep and expect link up from the back when there are literally 4-5 men in front of him.
A series of tactical issues, surprised he didnt change anything and only think it was a fullback issue. Clearly nothing much change when they came on.