That post Bayern period here lasted 2-3 games at most if I remember right.
1st round: Barcelona:Villareal 4:0
4231: left Fati-CAM Cou-right Griezz, Messi as a no9
2nd round: Celta:Barcelona 0:3
4231: left Fati-CAM Cou-right Griezz, No9 Messi
3rd round: Barcelona:Sevilla 1:1
4231: left Fati-CAM Cou-right Griezz, No9 Messi
And then Koeman started to rotate and change positions (maybe Messi himself asked to be returned deeper because he wasn't involved enough in buildup as a NO9 in 4231?), look at this:
4th round: Getafe:Barcelona 1:0
4231: Dembele, Pedri, Messi + No9 Griezmann
5th round: Barcelona:Real 1:3
442 Left winger Cou-Frenki-Busquets-Pedri, attack: Fati+Messi
6th round: Alaves:Barcelona 1:1
4231: Fati-CAM Messi-Dembele + No9 Griezz
7th round: Barcelona:Betis 5:2
442: Pedri-Frenkie-Busquets-Dembele + Fati-Griezz
8th round: Atletico:Barcelona 1:0
4231 Pedri-CAM Messi-Dembele + Griezmann
So, can we say that last season was Cou's fault?
He started as a CAM in the first 3 rounds and we won 7 points in those 3 matches with 8:1 goals ratio.
Then Koeman started to move Messi back to RW and CAM positions and started to give more time to Dembele and Pedri instead to Coutinho.
Result: we won 1 out of the next 5 matches. 1 win, 1 draw, 3 defeats. Goals ratio: 8:8
After that Koeman returned Cou in the lineup, this time as a CM (Iniesta) in 433 in 4:0 win vs Osasuna.
Only to change the formation again in the next match to 4231 against Alaves (1:2 loss) with Cou as a left winger in 4231, with Griezz as a CAM (wtf?), Messi RW and now Braithwaite as a NO9.
Then Koeman again returned to a good old 4231 against Levante at home.
Cou finally played as a CAM, with Braithwaite on the left (wtf?) and Griezz on the right and Messi as a No9.
We won 1:0.
In the next match against Sociedad, Koeman played the same 4231 but now with Pedri as a CAM and Cou was benched.
We won 2:1.
But in the next game against Valencia, Koeman played 4141 with Busi + Braithwaite-Cou-Pedri-Griezz + Messi.
We played 2:2.
So, is Cou guilty for this?
Koeman couldn't play two matches in a row with the same formatiion and a lineup.
4231, then 442, then 433, then 4231, then 442, then 4141.
And he moved players, for example: Cou: CAM, left wing, CM, LM in 442.
Griezmann started as a NO9, then moved to a RW, then to a CAM.
Messi was a No9, then RW, then CAM, then NO9 etc.
Cou played 3 matches in a row only in the opening 3 matches when we played well.
Then Koeman started to play more Pedri and Dembele and our results and faster played disappeared.
You can check our highlights from the opening matches when Cou was allowed to play as a CAM in 4231.
Barca:Villareal 4:0, assist:
https://www.youtube.com/watch?v=p9kzzmhUZOg
Celta:Barcelona 0:3, assist and a few nice shots:
https://www.youtube.com/watch?v=chBQpI103AQ
After that we played crappier with Koeman's random formations.
And the only time when Cou played well again was in CL matches when Messi was rested.
So, all in all, in the last season, you have at least 3 factors:
1. Messi was here, Messipendencia was strong
2. Cou played only a few matches as a CAM
3. he never played more than 3 matches in a row except in the opening 3 rounds when we played awesome
4. Koeman was constantly changing formations and switching player's positions
5. later he got injured and missed the most of a season