Dumfries recently signed a new deal to 2028.
Gullit said on Ziggo Sport: "I always thought that many people didn't believe in him because they thought he could only run.
"Then I see where he ended up... I have to say that he has learned a lot. And then he is very strong mentally, because he has grown and now he is one of the most important players in our country."
Dumfries, also speaking to Ziggo Sport, said of his new deal: "It took a long time. Of course we had to wait for the new owners to take over. We negotiated for a long time, but in the end I am proud and happy to have signed here.
"I think it was not easy for them and not for me either. That made things a bit difficult, it took a bit longer than usual. We can go around it, but in the end it is also a question of money."
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