Madrid have been heavily linked with a move for the 18-year-old after an astonishing 2016-17 campaign that saw him score 26 goals in all competitions for Leonardo Jardim's side.
Spanish newspaper Marca have claimed the Spanish and European champions have struck up a world-record €180million deal for Mbappe.
And Casemiro says, while he is aware of Mbappe's talents, he does not believe Madrid need him given Zinedine Zidane has the likes of Cristiano Ronaldo, Gareth Bale, Karim Benzema and Isco at his disposal.
"We're aware that he's a great player, but we've also got the BBC [Bale, Benzema and Cristiano], who can wreak real havoc," he told reporters.
"The more players we have, the better it is for us. That said, the best players are already here.
"We know all about the quality that players like Mbappe, Neymar... possess, but I've got to talk about our players.
"We've got the world's greatest player [Ronaldo], then there's Bale - who's working very hard - and Karim, who has great quality, Isco, Borja [Mayoral]…I'll back our players to the hilt."