“The incidents cut our legs off. We were surprised,” Mensah told Le Progres newspaper.
The experienced Togo international was right in the firing line at the start of the round of 16 on Sunday in Louhans.
The game was interrupted after just four minutes due to thick smoke on the field after an object thrown by visiting fans just missed his head and landed right next to it.
The hosts conceded two quick goals when the game resumed 20 minutes later.
“It was pretty chaotic,” said the 32-year-old goalkeeper.
“I heard a strong bang and felt a pain in my right ear. I got the impression that a bomb was going off and felt the pitch vibrate.
“The doctor explained to me that if it had exploded any closer I could have pierced my eardrum. It really shook me at the time.
“In the locker room the referee and match official asked me if I could continue and I replied yes and they decided to go back to the game.
“Psychologically, it wasn’t easy for the supporters to threaten to throw another banger. I tried to ignore it, but it stayed in the back of my mind.
“It’s a shame, it spoiled the party, but Saint-Etienne outclassed us on the pitch, there is nothing more to say about that.”
The French Football Federation (FFF) is investigating the incidents, which are just the latest in a long list of riots in France this season.