Coppola has moved to the Ligue Une club on-loan for the remainder of the season.
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The 22 year-old moved to Brighton in the summer from Hellas Verona.
Brighton manager Fabian Hurzeler said: "We're really pleased with how Diego has settled in and adapted to the demands of playing in England.
"His attitude has been outstanding and he now has an opportunity to be playing regularly in France before we welcome him back in the summer.
"Getting more game time will benefit his development and give him the best possible chance of helping Italy reach the World Cup finals and be part of that."