The 20-year-old Argentina international, a product of United's academy, is reportedly open to leaving Old Trafford this summer, with Chelsea and Napoli among the clubs showing interest.
Petit, however, has voiced doubts about the wisdom of such a move. Speaking to the media, the Frenchman stated that Garnacho "has not improved as a player" and expressed unease about the youngster's attitude, particularly during United's recent pre-season tour of Asia.
"Chelsea should steer clear of Alejandro Garnacho, he might be a decent guy but I didn't like his behaviour on Manchester United's tour," Petit told talkSPORTBET.
"He is showing frustration but has had a lot of chances to show his potential on the pitch, but still we don't know what to expect from him. He gives 100% and shows great commitment, but we haven't seen him really improve as a player.
"The mentality is there on the pitch but he hasn't been improving step by step. Chelsea already have wingers in similar situations with the same frustrations. Signing Garnacho is not a good idea."
There, Garnacho was seen swearing on camera and allegedly pushed a fan - behaviour Petit labelled as unacceptable for a professional.
Adding to the uncertainty is Garnacho's reportedly strained relationship with United head coach Ruben Amorim. Sources suggest the friction may be serious enough that Garnacho has already played his last game under Amorim's management.
Despite being under contract until 2028, Manchester United may consider parting ways with the winger - though only if their high valuation is met. Garnacho's academy roots could complicate a potential sale, as the club traditionally hesitates to let homegrown talent go cheaply.
Chelsea have made Borussia Dortmund winger Jamie Gittens their top transfer target this summer, as Enzo Maresca seeks to reshape his attacking options ahead of a return to the Champions League.
Newcastle United have emerged as frontrunners to sign Marcus Rashford on a season-long loan, aiming to provide the Manchester United outcast with a platform to reignite his career, according to reports from The Sun.
Chelsea winger Mykhailo Mudryk could face a suspension of up to four years after being formally charged by the Football Association with an anti-doping violation.
Manchester United's pursuit of Sporting CP striker Viktor Gyokeres has taken a positive turn following a major shift in Arsenal's transfer strategy.
