Marseille has little time to regroup with a league game at Ajaccio on Friday, followed by the return leg at Inter four days later.
Tuesday’s match was originally scheduled for early February but was postponed because of freezing weather conditions.
Marseille, which was missing top scorer Loic Remy and tricky winger Mathieu Valbuena because of injury, missed two chances to take the lead before Leroy’s opening goal.
In the sixth minute, Evian failed to properly clear right back Rod Fanni’s cross and the ball landed to Benoit Cheyrou, only for the midfielder to volley wide of the open goal from a good position.
Winger Andre Ayew’s pass then put defender Jeremy Morel clear with only goalkeeper Stephan Andersen to beat, but his shot hit the side-netting.
Evian took the lead in the 16th minute. Striker Yannick Sagbo passed to Leroy, and he shimmied past Fanni before expertly clipping the ball into the top corner.
The 37-year-old midfielder, who played for Marseille between 2000-02, has a habit of scoring against his former teams. He hit his second goal in the 22nd, beating goalkeeper Steve Mandanda at the second attempt after his first shot from close range was saved.
Ayew forced a smart save from Ayew at the start of the second half. But Mandanda was also called into action, punching away defender Brice Dja Djedje’s 30-yard shot.
Mid-table Evian should have made it 3-0 with 15 minutes left, however midfielder Cedric Barbosa unselfishly tried to set up Saber Khlifa and the chance went begging as the pass didn’t quite reach him.