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Green, 20, kept out a late spot-kick from Renaud Ripart after being named in the side following injuries to Jessy Moulin and Stefan Bajic.
Goals from Wahbi Khazri and Denis Bouanga secured a second victory in three game for Les Verts (the Greens in English) to lift them seven points above Nimes, who sit in the relegation play-off spot in 18th.
"It's a proud moment to make my debut for Saint-Etienne, I've been at the club since I was nine," Green told Canal Plus.
He was born in Colchester in southeast England but moved with his family to France as a child, and said he was named Etienne to remind his mother of home.
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"Yes, it has a link. I was born in England, my mother is French and my father is English. To remind my mother of France they decided to call me Etienne."
Green signed a professional contract with Saint-Etienne last year after coming up through the club's academy.
"His performance didn't surprise me," said Saint-Etienne coach Claude Puel.
"I knew he was capable of a match like that. He was huge today. He was calm and intelligent. Let him enjoy it."
Green added: "It's going to be hard to sleep but it's wonderful."
mw/dj