There was a moment where Messi made his typical run through to the left side, and clearly waited for Firpo to look up / be ready which he didn't/wasnt and Messi was intercpeted because of that. The commentator pointed it out as well. Point is: Firpo still lacks the understanding to play as a RB here in Barca.
That doesn't mean he is a bad player. With Betis his role was considerable different. Every mistake he makes here can have dire consequences, due to our slow midfield and players in general. We still lack a lot in the physical department and our offensive key players are suarez and Messi, and to maximize their output we need disciplined and clever players. Also why Arthur probably has difficulties, and Rakitic still can be a valid options despite his slowness and rather bad technical ability.
Firpo is now at stage where he is to focused to not make mistakes, and try to find his rhythm into the team. That's why he isnt showing his best version - yet. On the plus side, he looks to have both the technical and physical attributes to succeed here, and diciplin and understanding of the tactics and flow of the game can be learned way easier than getting faster or learn to do 1 vs 1 situations. With time and maybe a better manager he has the chances to succeed here (injuries are still a big concern tho).
There isn't really a reason to be to concerned about him. He has been okay. and the game before atm, he was really good actually. It's also not like atm is a bad team. Well at last note defensively he might still be lacking, but so did Alba at first.