Barcelona does have a bad defense, conceding 36 goals from 36 appearances is something to watch out for. I agree Koeman must indeed
improve Barcelona's defense in the next season, otherwise they will be difficult to compete with other top teams. Barcelona must buy quality
defenders, even though Barcelona are experiencing financial difficulties, they could sell some of the players who didn't contribute this season.
Their defense system is never strong even with Pep era. They had less conceded goals in Pep era because their tiki taka dominated matches and did not give opponents any chance to counter attack. After the golden era with Pep, Iniesta, Xavi, Messi, etc. their tiki taka became weaker and their defense system became more vulnerable to counter attacks.
Barcelona's midfield and forward are strong enough at the moment. They have a lot of young talented players who are talented but not experienced enough yet. If they become experienced, Barcelona will become irresistible. Now Koeman should build young talent properly and strengthen the team's defense. Barcelona should now include some skilled defenders in the squad.
Koeman is a legendary player in Camp Nou and he is a defender. His philosophy as a coach is different than Pep. He is not obsessed by tiki taka or ball possession.
The defense system of Barcelona will hardly be improved if they won't have contract for this area. It is wasteful of transfer budget as in recent seasons, they invested too much for defenders but most of them perform less than expected. Back to general transfer of Barcelona, they overpaid for attacking players too, not restrict to defenders.
With the return of Laporta, I hope their transfer strategy will become wiser and will help them rebuild their team. The era of Messi is ending but Barcelona still can rely on Messi contributions in a few seasons ahead.