Referee Lionel Jaffredo sent both teams back to the dressing room after the 31st-minute incident, and the match was abandoned more than 45 minutes later.
"Public authorities have taken the decision, in agreement with the match delegate and the referee, to call off the game," the French League (LFP) said in a statement.
Metz were leading 1-0 thanks to Gauthier Hein's goal two minutes earlier.
Lopes fell on to his back, with his hands on his ears, when the first firecracker was thrown and had to roll out of the way when another exploded at his feet while he was being attended by medical staff.
Lyon said Lopes was taken to hospital for medical checks.
"Anthony Lopes is injured, and so is the doctor. At best, Anthony experiences tinnitus, meaning he loses his balance and does not really know where he is," Lyon president Jean-Michel Aulas said.
"Worst-case scenario, his eardrums are damaged."
(Reporting by Julien Pretot; Editing by Neville Dalton)
