The former Manchester United forward has been outstanding for the past week, with De Zerbi full of praise for Greenwood on Friday.
"We always need a Greenwood like the one we saw in the last two matches," said De Zerbi.
"He's a world-class Greenwood because, in addition to what his mother and father taught him, he's becoming a complete player.
"He's also had his share of difficult times, but he didn't leave and he stayed here. He understood that we were telling him things for his own good and for the good of Olympique Marseille."
The Italian added, "He's truly become a player.
"I hope he continues like this. I'm threatening him with that, assuring him that he has to keep going like this and that it's primarily for his own good. Because I also see him as a little more human.
"He smiles sometimes, he talks a bit more. He's less withdrawn than before. And that's a positive thing. Now everyone can really appreciate Greenwood even more than before."