"I have never really had that much trouble getting used to a new competition or new team so I hope I can do the same again," the Holland international told the club's official website.
"It was great to go to Portugal for my development and I am sure that I improved a lot over there in the two years.
"During that time I of course played a few games against English clubs in the Europa League and scored against English teams, so I know what I have to do and I know what I can do in the Premier League.
"I really hope that I can play my game and stamp that on the Premier League. It is a perfect move for me."