Wenger hardly needs a tip to ignore off-field tactics from his Barclays Premier League counterparts, but his former Monaco and Arsenal midfielder is offering it anyway, saying Mourinho's recent claim that he will field a second-string Chelsea against the Gunners in their Capital One Cup clash at the Emirates on Tuesday may not be true.
"Do you believe Mourinho?," he asked rhetorically while in London to preview the clash.
"I think he's a competitor, he wants to win every single game. It doesn't matter if it's the Capital One Cup, if it's the Premier League or Champions League, whatever, even if it's a friendly game he will want to win it.
"Probably he will make a little bit of rotation in the team because they will be playing two days before the game, but if you look at the squad with Chelsea, there's a lot of young players, so what does it mean if he says he will play a young squad."
Wenger conceded after Arsenal's 2-0 win over Crystal Palace on Saturday that he too would rotate his squad, with Mikel Arteta set to miss out with a one-match ban - although he was unlikely to play regardless - while other players will be rested.
Petit, meanwhile, argues Mounriho's young squad, all vying for attention from the manager, is another reason for change.
"If he wants to make the dressing room happy, he has to give the time for players to show what they can do, what they can bring to the team," he explained.
"If you only base your expectation on the first 11 then for sure you won't win anything at the end of the season.
"It's not only the 11 players on the pitch, it's 25, sometimes 30 players in the group, so you have to make a rotation to make everybody happy.
"But as well you have to do that to keep the freshness in the team, in the spirit as well. Mentally, physically, it's important," he said.
The match comes during a tough month for Arsenal, who host Liverpool at the weekend, before their pivotal Champions League clash at Borussia Dortmund and then Premier League matches with Manchester United and Southampton.
But Petit believes, despite Arsenal's Champions League campaign suffering a 2-1 loss to Dortmund last week, this is the season the Gunners will end their long spell without silverware, with the help of a young German and a returning Frenchman.
"It's time now. I was a bit worried at the beginning of the season because I didn't see big guns coming to the club, but then they put £50million on the table to bring this wonderful player Mezut Ozil," he said.
"(Mathieu) Flamini as well, nobody talks about Flamini, he's come back to Arsenal, but I think it's very important ... because he knows very well the club and in some ways he knows, he has some revenge as well with the club.
"Everybody knows his physical potential on the pitch and his winning mentality.
"The only problem with Arsenal I can see is if they can avoid many injuries. It has been a very big problem in Arsenal's dressing room. But their first 11 is very strong, their confidence and happiness when they look at the game, it's amazing."
Petit lifted the league title and FA Cup in his first season with Arsenal, after building a formidable midfield partnership with Patrick Vieira, and he now sees something special in the Gunners again, although not just in the centre.
"The way Arsenal is playing actually the confidence they're showing on the pitch, I haven't seen that often, except maybe Barcelona or Bayern Munich," he said.
"I haven't seen real confidence like this 100 per cent confidence (before with Arsenal)."
He added - as Wenger continues negotiations over a new contract - the need for another trophy in the cabinet has never been greater.
"He needs to win titles especially domestic competitions, like for example the Premier League," Petit added.
"The national cup as well is very important because most of the time they are the beginning of the rest, so that's why this game, the Capital One Cup is very important."
Petit has not ruled out Chelsea's chances of taking the match on Tuesday night - although he does admit his heart is with Arsenal - but he is sure it will be one of the most competitive games of the season so far.
"It's the derby and honestly, I used to be a player, it doesn't matter which competition you play, you have to win the game because this game can have an effect on the rest of the competitions, especially the Premier League," he said.
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