The retired midfielder is disappointed with the phyiscal and mental preparation shown by Les Bleus so far and urged the players to show their true colours against Spain
After securing four points from their first two matches, France finished second in Group D after suffering a shock 2-0 defeat against Sweden.
It ended the team's unbeaten run of 23-matches, and Petit thinks it was a reality check ahead of the knockout stages.
"I did not expect to see France end such a run against an opponent who were already eliminated," the 1998 World Cup winner told France Football.
"We can lose, that is not the point, but we cannot lose like this. Even if they tried, I do not think they were physically or mentally ready to face Sweden."
The 41-year-old, who retired from football in 2004, then admitted that there are a number of players who are under-performing for the national team.
"The defeat confirmed defensive and offensive problems," Petit added. "Philippe Mexes is suspended, Laurent Koscielny will fill in for him. Could this be his chance? I am not sure.
"Nowadays, the great majority of players in France's squad are on the same line.
"If they want to go far in this tournament, they must realise, like Yohan Cabaye has done, that the team's fate will also depend on individual performances.
"There are several players who are not playing at their usual level. Sooner or later, and not later than the match against Spain, they will have to show what they can do."
Petit also said that he thinks coach Laurent Blanc has become too predictable and that the 46-year-old should think about introducing different players in the second half.
"Laurent Blanc has some small coaching problems," he explained. "This is especially seen in the second half, where he is slow to act. He almost only trusts the same players all the time.
"He always he goes through with his convictions, but he should perhaps reconsider his choices.
"I am talking about players like Olivier Giroud, Mathieu Valbuena and Jeremy Menez because we are missing some offensive power.
"This was seen in the match against Sweden."