You say that as if most Championship players had already spent a season (at the age of 20) somewhere else in Europe (with a different style of football) as the best player in the league. He has shown incredible ability even if it was in a weaker league, and he has shown glimpses of it here. People underestimate the importance of confidence in sport. Some people are more fragile than others - a manager that actually has confidence in him (not dropping him after a bad game when Rooney, Lingard and Fellaini are all exempted, or actually allowing him to take risks) could make him rediscover his old form.
Some players reach their potential later than others, not everyone does a Ronaldo or Messi. Some players also take longer and find it harder to adapt to a different style of football. He's also not had the easiest life, by all accounts, and this is the first time in his life that he is living away from home. He's only 22, that's university age. Too many people forget that footballers are human beings and have regular day-to-day things that can affect their performances. He has great potential, he just needs to not let this opportunity slip - you won't stay at a club this size for too long if you don't show you are good enough.
@K2K True - no one is doubting that he was subpar last year. I am a big fan of his and will be the first to admit he was more often a hindrance than anything else most of the time, bar a few games early in the season and the time we beat Arsenal in the run in. Still, Van Gaal made players like Di Maria look average - surely it's obvious that players that play very risky football rarely thrive under him (at least from his Bayern days and after).
The most notable exception is Robben, who happens to be a winger who can play up front, and is also excellent at keeping possession (two things LVG is obsessed with in a wide man, seemingly).
I'm not saying what he has shown last season is enough - I am saying that I still think he is going to turn it around. He seems determined and professional. He's young and has time on his side.