The Chilean has struggled at United since joining from Arsenal in January.
And Petit has joked that the Red Devils must have signed Sanchez's brother.
"The manager is on the bench, so you need to show pride in your own performance when you are on the field and the first one who has gone into hiding at United is Alexis Sanchez," Paddy Power ambassador Petit told Goal.
"He wanted to leave Arsenal because he was chasing the salary United could offer. Now he receives so much money and in nine months what has he done? Nothing. I have not seen him. I think they signed his brother.
"I understand that players believe they deserve certain financial rewards when they reach a certain level, but what we have seen from Sanchez at United has not been acceptable."