Greenwood has now scored six goals and provided one assist for Marseille since joining the club from United.
His latest outing resulted in Montpellier sacking coach Michel Der Zakarian, as president Laurent Nicollin confirmed after the match.
"Just to tell you that I have terminated Michel De Zakarian's contract," he said. "He is no longer the coach of Montpellier. Given the performance and the attitude that I saw this evening, I felt obliged to intervene and end our collaboration. It is not with a light heart, because he is someone that I appreciate enormously. But in life, it is the club before everything. There is nothing planned for the future at the moment. Nothing was anticipated. I am ashamed to have taken a 5-0 loss against Marseille, for my club and my supporters.
"I told my players this evening, they got rid of the coach. It is up to each of us to realise that if we do nothing, we are heading straight for there... Something had to be done. That's what I feel, I hope I'm not wrong. It's never easy, especially when you like people. But that's the life of football, when there are no results, it's easier to change a coach than 25 players."