Karanka was Mourinho's No2 at Real Madrid for three years.
He said: "For me, to spend three years working with Jose was the best possible start.
"He has supported me for the last month and wondered when I would arrive.
"I have been in contact with him 10 or 20 times and now that I have arrived, he will be happy."
Karaka added: "I'm Karanka - there is only one Jose Mourinho and he is the best. My character is very different. I am calm but the most important thing is to be a leader in the dressing room. The players have to believe in me."
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