Greenwood scored twice and created the other as Marseille won 3-2 at Champions League opponents Union Saint Gilloise.
The former Manchester United forward was named man-of-the-match and De Zerbi said afterwards: "He's one of the best strikers in Europe, but even he, when we're leading 3-1, needs to learn how to control the ball, not force it, and not lose it.
"It's not an excessive requirement, but Greenwood must also help the team defensively, holding his position and giving the rest of the group some breathing space."
Greenwood's Marseille teammates also heaped praise on the forward - though with a qualifier.
Pierre-Emile Hojbjerg said: "Mason does a lot for us, but he can improve his consistency until the end of the match."
Goalkeeper Geronimo Rulli stated: "He's one of the best on the team. I know his value; it's a special night for him."
And Geoffrey Kondogbia added: "He's our key player in attack. Our job is to put him in the best possible condition, both tactically and psychologically, to allow him to continue making a difference."