Ferguson slates Gerrard in his new autobiography.
Zidane, now a Real coach, said: "Gerrard has been loyal to Liverpool throughout his career - which is great for them - but if he had wanted to leave four or five years ago then every top club in Europe would have tried to sign him.
"And they would have probably had to break the transfer record to do it.
Zizou added: "For two or three years, Steven Gerrard was the best midfield player in the world.
"Even now he is playing at a high level for Liverpool and England."
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