I find this hard to believe.
First. Barkley is playing in a highly competitive professional sport, is extremely well paid and is regarded as a player with quality above the ordinary. He is an international player where the standard is even higher again.
Are we to believe that he cannot cope with the demands of the sport that he has chosen as his career?
Has he spent his entire career surrounded by coaches offering nothing but praise until Koeman arrived.
Can you imagine then how managers like Pochettino, Conte and Wenger would assess news like that ?
...it's always been my assumption that he was the centre of the Everton universe under Martinez, given little or no team responsibility when the opposition had the ball and constantly described as a phenomenal talent. That changed under Koeman because he wants more from his footballers. These lads have enormous belief and Barkley's ego has taken a bashing. He's no longer Everton's main man and has been discarded by England.