The 20-year-old midfielder has shocked club officials after revealing that he has no intention of signing a new contract with the French club, leaving manager Claudio Ranieri frustrated and disappointed by his decision.
"I do not understand his attitude," said the Italian. "As soon as I saw him play, I said here is our Makelele. I trusted him and I played him.
"I think he will miss out on a great deal. But it's his life, not mine. He has preferred to listen to his brother and agent and not his coach. It's a shame."
The France U21 international is believed to have had his head turned by alleged interest from Manchester United, with new manager David Moyes ready to lure the new Claude Makelele to Old Trafford this summer.
Mendy made 31 appearances last season helping Monaco secure promotion back to Ligue 1, but has decided to turn his back on the club that gave him his first-team breakthrough, requesting to leave Stade Louis II when the transfer window reopens after rejecting the offer of a new contract.
Manchester United are ready to pounce by luring the player to the Premier League, and will have been impressed by his recent performances in the U20 Toulon Tournament, where Mendy helped France to a third-place finish on Saturday following a 2-1 victory over Portugal.