Hiddink revealed "Arjen came to us from the north of Holland as an innocent boy two years ago and within four months of getting used to the training and games, he made the kind of progress I have seldom seen from any young player in such a short space of time.
"He was very open and coachable. You can give him some advice or train two hours in the morning and evening and he will do it.
"His mind his open. That is why he made such tremendous progress and he can continue that at Chelsea.
"I did think it might be better to stay with us for another year but you have to give him the benefit of the doubt because his personality is so strong."