"What people seem to forget is that it really is a very big step to come from Holland and the Eredivise league there," said the Dutchman.
"It's a huge step, and it's really difficult to be a success straight away. The style of football is different, and the pace is different too.
"He has done brilliantly from the very beginning, and he has become a very important player for Liverpool, but it is my belief he will be even better in the future.
"He's a great player already, but it still takes time to settle in, coming from Holland.
"He's still young so he has time on his side to get used to new teammates, and to get used to the Premier League properly."