I understand U, but maybe he would addapt to our style after some time... we cant't just dismiss players by denying their addapting abbility... he is young, he can be sculpted by manager and our school and players into what we need him to be on the pitch...
Ok.
Every player has some room for development/learning/changing his style.
But also, every player has some natural talent/footballing personality. Depending on that natural talent/personality, a player can improve to some extent, but an improvement is related to his natural levels of each skill.
Example, if a player is naturally slow (his pace is 50 points out of maximum 100 points), then with training, he can improve to 60-70, but he will never be as fast as a guy who was born with Pace 90 and who trained his pace to 100/100 level.
Or, if you are a natural-born dribbler like Messi, Neymar etc, you will only improve that skill over time.
On the other hand, Rakitic isn't naturally as good in dribbling as Messi, Neymar or Iniesta.
Again, he can improve over time in that area but he will never be as good as Iniesta in that area.
About Coutinho or any other player with a limitation is some skill/area, you always have to think about his natural talent in that skill.
So, his natural talent/footballing personality is a base, and after that, you can improve 10-20-30% in that area (on your base skill/talent).
Of course, there will always be one example or one player (1 out of 100) who was very poor in some area as a 20 year old and turned into a beast later.
But (imo), majority of players are somewhat predictable in their development if you "calculate" their natural skills and a % of possible development in that area over time.
To go back on topic, about Coutinho, and any other young player, he will surely improve in areas where he is weaker today, but also, each improvement (for 99% of players) has (a somewhat predictable) limit.
Imo, the other guy, Firmino, works waaaay more than Coutinho (even though Coutinho has more technical talent) and seems to be more suited for our style than Coutinho.