The former Ajax star has battled for consistency since joining Barca 18 months ago.
Koeman said, "Frenkie is a very good player. He sees everything that happens on the pitch, always makes the right decision and, above all, is always in the right place.
"Last season he didn't play his ideal role. When young it is always difficult to come to Barcelona: there is another language, another way of living and training, but now I will give him all the help he needs and let him play in his role. I'm not in doubt."