What is Ronald Koeman doing wrong tactically?

Though Barcelona won all the initial matches in champions league and 5–2 against Betis but there are many flaws in tactics and substitution.

Prssing game- Coming on to pressing game which koeman insists heavily on the team, sometimes pressing as a whole, Barca’s Defence goes on sleep and results in error in marking opposition, and when opposition put a bit of pressure barca’s Defence falls like a mud wall, i can understand that it's a new team and Barca lacks in defenders but playing with the same back 4 which led to a thrashing defeat against Roma, Liverpool and Bayern and now again this new coach is going on the same path down. Lenglet is not a world class defender and commits lots of error he should be immediately replaced by Araujo coz atleast he does have pace to outclass and pacey forward and he trackback easily, Pique should be replaced by Eric Garcia in the coming January hope Barca sign him .

Improper substitution- why on earth De Jong never get substituted when he has a bad game and atleast for 5–10 mins, he can be substituted to give chances to other players to get some competition which will indirectly improve his results.

Busquets , i know many people will start Pjanic instead of Busquets, but Busquets is not bad either he looks vulnerable because of Barca's shitty defence he should be given 45 mins to play at his full potential and then subbed off by Pjanic for another 45.

Playing Roberto knowing how weak he is defensively, everytime ball went to right flank he looked vulnerable, some may argue that he was good against Betis but he is only good with Messi instead he shits, he should be replaced by Dest , koeman goes in comma when comes to like for like substitution , he always looked in hurry to put forwards during substitution idk why.

Key point- Throw Lenglet (he does not have brain, put Araujo instead)

We need atleast 2 Defensive ace in this January transfer window, forget number 9 Barca can play without that but without good defenders Barça will be humiliated once again only days are numbered.