The former Chelsea winger said: "Klaas is a goalscorer and I am sure that he will score many in the Real Madrid shirt. It will be very different for him because he has only played in one league in Holland and that cannot be compared to the Spanish.
"However, I think he will adapt and will do very well here."
On former Portsmouth midfielder Diarra, Robben added: "He is a very good boy and I personally like his signing. I played with him at Chelsea and he is a player that has several similarities to Claude Makelele.
"He is very strong defensively and does a great job. I was always playing with him in the Chelsea dressing room and we have very good group here."