The Holland midfielder is the subject of a €55m offer from Manchester City, though the Rossonero are so far refusing to do business.
But Gullit said of his countryman: "I'm very happy with his path. Three or four years ago he played with my son and when (Arne) Slot sent him to the (Feyenoord) second team Reijnders got very angry.
"Now he has had a very rapid rise. He is a strong player, who knows how to score goals. It will be an incredible loss for Milan when he leaves. He is someone who wants to win, he is not satisfied with staying in a club that doesn't win."
Meanwhile, Gullit also said at the 20th edition of the Vialli and Mauro Golf Cup: "It's a shame that Milan has slipped so far down the standings.
"I hope they can find a formula to climb back to the top because when you see the team you can't think it's bad. The players aren't bad, I don't know what's happening."