The Ligue 1 clash against Lyon was abandoned in the 31st minute following trouble from the stands.
The decision means the stand behind the goal at the Stade Saint-Symphorien, from where projectiles were thrown at Lopes, must remain completely closed until a final decision on eventual sanctions is made at a later date. Metz risk forfeiting the match.
Metz had anticipated the move earlier in the day by announcing that they would close the same part of the ground of their own accord "until further notice".
The Ligue 1 clash against Lyon was abandoned in the 31st minute following the incidents, which came after Gauthier Hein had put Metz 1-0 up with a superb goal.
Portugal international Lopes was subsequently taken to hospital where he was diagnosed with "acoustic trauma", although he was able to play in Lyon's 0-0 Champions League draw with Sevilla on Wednesday.
"The incidents that took place last Saturday at the Stade Saint-Symphorien during the match against Lyon are inadmissable and proved that a red line had been crossed," Metz president Bernard Serin said in a statement published on the club's website.
"That is not what football is about and nor must it become that way. That is why I wished to take measures to avoid any chance of these events taking place again," Serin said of the club's own decision to close the stand behind the goal.
A 23-year-old man was still in custody on Thursday after being arrested on Wednesday in connection with the incidents. He has admitted throwing one of the firecrackers, the Metz public prosecutor said.