The former Ajax captain has had a slow start to his Premier League career.
“He needs time," said the Blues boss. “He is okay. He is intelligent on the ball.
“I think he can improve in fitness and he can improve physically in battles in the midfield.
“That is what he needs to learn and improve, but that is normal.
“The Dutch league is not the Premier League and the boy gets time for that. It is all about the player himself."