Former Manchester United forward Greenwood struck the decisive goal - a week after being hooked at halftime in defeat to PSG and spending time on-one-on with his manager.
De Zerbi said after the win: "I was surprised last week when he didn't play well, at a level that is not his.
"Mason is a very good boy, above all else. He is committed, he gives his all. He is a strong player, who makes a difference with OM."
OM teammate Adrien Rabiot also said: "He has potential, we knew that. What we need is for him to maintain this rhythm and not alternate with matches like last Sunday. There is something to do, something to score and make passes. There is a lot of talent in this team, we need to show a certain consistency in the championship, that is what will be important."