Portugal have confirmed that Ronaldo is suffering from tendonitis, leaving the side’s captain as a doubt for their World Cup opener against Germany on June 16.
"I'm concerned that a player might get injured as happened with Kaka or with [Sami]Khedira with Germany," Perez told Cadena SER's radio programme, 'El Larguero’.
"Players like Cristiano should only play when they are fit."
Perez then went on to dismiss any current Real Madrid activity in the transfer market, but did reveal that Iker Casillas is set to stay at the Santiago Bernabeu along with Alvaro Morata, who has been linked with Arsenal.
He said: “We haven't made any moves in the market as yet. We'll have a talk with Morata, but we haven't sat down together, yet. I am a friend of the Juve directors, but I haven't spoken with them. The chances of Morata going there are nil.
"I had breakfast with Casillas and his agent. He has never said anything to me about wanting to leave. Casillas has been at Real since he was nine-years-old and as far as I know wants to end his career here.
"He'll see out this year at Real Madrid. Contracts are to be honoured, he has a buy-out clause."