Karanka won three Champions League titles with Real Madrid from 1997 to 2002.
"I was not at all surprised by the change, but perhaps a little surprised by how quickly he has done what he has done," says Karanka to The Mirror.
"I played with Figo and Zidane, and even for them it took time. I remember when Zizou arrived, he wasn't as good in the first month or two as he was at Juventus.
"But with Jude's personality and character, it seems that he has played at Real Madrid for ten years. No other club is like this one. When I played there, there were top players, but they were all older. Now they are 20 years old.
"The only one I can think of is Raul, who started at 18 or 19, but when you're as young as Jude, it's hard to find someone who has had as much success as him."