Left-back: Jordi Alba

As it stands, Jordi Alba is undoubtedly among the best full-backs in the world, let alone in La Liga. In the years gone by, he has established himself as a regular in the starting lineup at Camp Nou.
Alba averaged 1.2 tackles, 1.7 interceptions, 1.6 clearances and 0.8 dribbles per match having made 36 appearances in La Liga in the 2018/19 season. He has been an instrumental presence further up as well, registering 2 goals and 8 assists.
Right-back: Nélson Semedo

When Dani Alves bid adieu to Barcelona, it was a topsy-turvy scenario at the club. Sergi Roberto filled in for the Brazilian and though he did an excellent job, he isn't a natural right-back after all.
Now Nelson Semedo has not exactly taken Alves' place, but the right-back berth is not the greatest of concerns at Barcelona. The Portuguese has indeed made great strides so far and will continue to improve with time and guidance.