Klaassen has flopped at Everton since in his move from Ajax last summer.
Former Everton manager has told Ziggo Sport on Sunday, as quoted by Football Oranje, that he made a mistake when signing Davy Klaassen from Ajax.
The 25-year-old has managed just six minutes of Premier League football since October, coming on as a substitute against Brighton in March.
The Holland international was signed for a whopping £25million by Koeman back in the summer, when the now-Netherlands manager was in charge at Goodison Park.
Koeman believes the lack of a centre-forward, following the sale of Romelu Lukaku to Manchester United did not help Klaassen but admitted that the jump from Dutch football to the Premier League proved to be too much.
“I called him a few weeks ago,” Koeman said. “He was in the picture again at Everton, but now again not. I do not know why. He had a difficult start at Everton, that was disappointing. But you can not say that he does not train hard.
“He had trouble with the aggressiveness and the tempo. You do not get time on the midfield. I was disappointed with Davy. What did not help him was that we did not have a striker.
“I really expected him to adapt more easily. That’s why I had also met him. Nobody in the club had doubts about him. Not in the Netherlands either. Klaassen was someone we spent a lot of money for and who could have really played a role in the midfield. In retrospect, you may have to doubt whether the step was not too big. “
In total, Klaassen has managed just 14 appearances, only five of which have come in the Premier League and he is likely to be transfer-listed by Everton this summer.
The three-time Dutch champions has four years still to run on the five-year deal he signed in the summer, and Everton may have to loan him out with plenty of Serie A sides being linked to the midfielder’s signature.
Receive exclusive football transfer news and updates twice a week to your mailbox
