Greenwood remains persona non grata for the England national team despite his strong performances in Ligue 1.
The Italian coach insisted that he is "a very different person" from the one portrayed by the English media.
"I never get involved in people's private lives, not even my own children's. But what I can say is that he's a good person," said Roberto De Zerbi at the press conference.
He believes that Mason Greenwood "has paid a heavy price" following the revelations from his partner, who posted images on her Instagram account showing evidence of domestic violence and attempted rape.
"He has come into an environment that suits him, where he has been shown affection and supported. He has behaved well, too.
"He is a reserved, introverted character. I know his family very well: they are good people. When I look at him, it saddens me what has happened to him, because I know a different person to the one described in England," the Italian coach said.
Before De Zerbi spoke to the press on Monday, it was Angel Gomes, the England international who played alongside Greenwood at Manchester United, who was asked about his former teammate: "It all depends on the environment you give him.
"He's a really nice guy. If you ask any member of staff, when he's on the pitch, he's enjoying himself. As a teammate, it's easier. As a friend, it's even better."