I don't think its so much about him adapting to the English game, as he has been wank in Europe as well. I think that we just don't play to his strengths. I had watched him a few times last season and he seemed a very good player. But in all the matches I watched, Ajax played possession football. They did a lot of small passes, a lot of 1-2s, they had a lot of movement off the ball and Klaassen was at the heart of it.
I'm not sure if Koeman bought him thinking that he wanted us to play like this only to bottle it when he saw it wasn't working, or if he thought that Klaassen could adapt to a different style. But if Sam wants to carry on playing the way we are then the only choice is to let him go somewhere else. He'd be brilliant at Napoli as they seem to play a faster version of what Ajax where playing.