coutinho is crucial to the system.. when he plays side by side with Arthur they produce that xavi iniesta effect of maintaining the ball. thats why arthur plays so close to coutinho in the pitch. this partenership is basically Valvderde.
Naturally this is a valid observation. But:
- The Xavi-Iniesta duo and effect represent the midfield. Arthur is still far from being a Xavi but apart from that the partnership only works if Coutinho plays in midfield. Often he does, even when in theory he plays as the 3rd attacker.
- When Coutinho plays well with Arthur (two Brazilians understand each other a tiny bit better) then actually a 4-4-2 is played, something that EV loves and would prefer, but it is far far from being a typical Barca style.
- When Suarez is sloppy and imprecise (as he is most of the time nowadays) that means Messi is almost alone does the attacking part, the creating part and the finishing part. So the "partnership" only works with a defensive setup and four midfielders.
- Therefore Messi needs an Alves-type help on his right to do dangerous things, but there's no Alves there, so Messi doesn't get those assists anymore, rather tries the known cross-balls towards Alba, that works fine but not as efficiently as in Alves's time.
- Roberto is a bad defender, a better midfielder and creator but the entire right-wing is his alone, which is way too much for his capabilities and talent.
- I don't dare mentioning a Dembele here (specially that you really dislike him) but the team has a very weak right wing, and a left wing that depends on Coutinho, who is NOT a winger, on Alba, who does well there but is not a winger once again.
Therefore the entire STRUCTURE is questionable, because just as you say the Coutinho-Arthur pair should play together but in that case whatever one dreams of and whatever results happen, it still remains a pseudo 4-4-2 with one single CREATOR and two attackers, both above 30. One needs to play a semi-creator and the other one is able to handle one ball in three...
There is almost no SYSTEM whatsoever and that is why results fluctuate and will keep on fluctuating until another coach arrives with some imagination...