Here is my take on Barkley.
Burst onto the scene the season we finished 5th and looked like a potential world beater. Our very own poster boy, who might actually stay and replace the disappointment of Rooney leaving so young and also become our own (and I don't even like saying this)...Gerrard, midfielder with ability, lots of magic moments, passion and a blue to boot.
The last two seasons of being coached by a clown and his clown coaching staff may have had a big effect on the lad, though statistically he is not doing too bad with assists and goals for last season, he still cannot stamp the bad things out of his game which by now you would of hoped he would do.
He is still grossly negligent and loses possession or gives the ball away far too easily and Koeman saw that against Sunderland. Whether this is down to concentration, sloppiness or confidence, he needs to get it out of his game and sooner rather than later.
If he cannot lose this from his game then he will never be any more than decent, which is still probably good enough to earn a position in a PL side, but certainly not the player he could have been, and if that happens he will go down in the list of massive disappointments of players who did not fulfil their potential in the shirt.