The Dutch midfielder said: "He is a threat. Sometimes I have to play right full-back in training, and I will tell you now, I do not like playing against Stewart Downing.
"He is so quick. He is probably the most developed player since I have been here - when I came here, he was a very young boy.
"The guy is very down to earth, he works extremely hard. We always look at his stats and we sometimes think, 'How does he do it?'
"He will run 13km on a Wednesday and then do the same again on the Saturday. You think surely he must get tired sometime, but he doesn't.
"I am happy for the club that they have managed to keep him here."