Klaassen struggled for his season in England before leaving for Germany to rebuild his career.
He recalled: "At Everton, the footballing aspect was also the starting point at first, but if you don't run twice as hard, the ball just goes up in the air and it happens in a different way.
"In the beginning I really thought: Yes, but guys, we can still play football and with power? But the rest thought otherwise, and as a player you just have to let it go.
"I really learned that in England, the different way of experiencing how to achieve a result, and it helped me a lot in Germany."