Barcelona looked like they had a great defending in the La Liga by conceding only 20 goals in the whole season. They could have even conceded less than that but they became relieved quite earlier about winning the league title so they started to care less about the remaining matches. However there wasn't the same look in the Champions League as they had serious struggles.
But now they have started to make the defense line younger. Because it wasn't the kind of a job that they would have done with old defenders like Pique and so on. I think the situation with centre-backs looks good for now. But I also agree that they had better sign a right-back especially. Because there is only Sergi Roberto there and they would have bigger difficulties in case of an injury.
When it comes to left-back there is already Marcos Alonso and Alejandro Balde is also a new internal transfer who is 19 years old. But if they don't want to continue with Alonso then they can look for a left-back as well.
It is true that as soon as Barcelona won the league, they became less careful about their post and the goals they concede. What they only cared about was for Lewandowski to remain the highest goal scorer as Benzema was following him behind with few goals difference.
It is nice that the old legends of Barcelona are leaving, my fear is that they shouldn't leave with all.tje experiences and allow the club to suffer experience.
I think the fact that they had such a great defense even for that long was something unexpected by many. Araujo was a "decent" player last season but wasn't this amazing, he was near perfect this season, Kounde was new and didn't expected to fit this perfectly neither, Ter stegen is getting older, Balde a youth player that nobody expected to even play this many games let alone be good, all in all there were nobody that really shine through the list and yet they played amazingly well together.
Rest of the team wasn't all that bad neither, maybe UCL happened early enough on the season that they weren't used to the tactic yet, if they can keep the core of the team together they might be a lot better next season, not expecting them to win UCL but at least will be out of group stages this time around.