The former Real Madrid coach has long been linked with Erik ten Hag's job at United.
But Petit told the Manchester Evening News: “I'm not sure that he speaks very good English as well. And communication is very important in the dressing room. Honestly, I know a little bit, Zizou, and he needs to get guarantees if he wants to sign over there in Manchester United.
“At the moment, the environment at Manchester United for years has not been good. The stability of the bench, the level of the players as well, the expectation of the club is not the same that it used to be. United is still a huge club, but not on the pitch anymore.
"And Zizou is very conscious and very careful about that, the quality of the players. At the moment, when you look at the team, I'm not convinced at all about the level of some players. If he takes the job at Manchester United, that means he's got solutions."