Dylan Batubinsika's fourth-minute own goal started a debacle for the visitors as Nice doubled their lead three minutes later, with Youssoufa Moukoko's brace making the hosts this century's first team to score six goals in the first half of a French top-flight game.
"There's no analysis. It's a disgrace what we did. We should be ashamed for ourselves, for our supporters and for all the work we put in," Petrot told DAZN after Saint-Etienne suffered their fourth loss in five league games this season.
"To concede eight goals, to be nonexistent in our state of mind and in duels. I don't know what to say."
The win coincided with Nice's 120th anniversary celebrations, with head coach Franck Haise saying the result would remain engraved in his mind forever.
"Matches with such scores do not happen often, if ever," Haise said amid a festive atmosphere at Stade de Nice, with the hosts claiming their second win in five Ligue 1 matches.
Petrot said Saint-Etienne would try to put this result behind them as soon as possible, with their next match at Nantes coming up on September 29.
"We've got 10 days to work really hard... We'll be looking ahead to the next game very quickly," Petrot said.