While Perez says his departure midseason would be "difficult", the president could not definitively say such a shock sale could not happen.
"It is very difficult for me to see Casillas leaving in December," said Perez. "For all he has done and his history, he is the best goalkeeper in the world.
"We have two great goalkeepers at the club (Casillas and Diego Lopez).
"Sure, there will always be fans who support one or the other."