The former Real defender has been plucked out of Spain's youth coaching system to return to Real.
"It was a huge surprise because when I got a call from Fernando Hierro on Friday I thought it was just a joke, but then he told me that it was true," Karanka, who won three Champions League titles with los Blancos, told the club's official website.
"We talked about it and he told me that he had spoken to Jorge Valdano, who said he wanted me to be an assistant to Mourinho. I spoke to Valdano later that afternoon and everything came together the following day."
The 36-year-old went on to reveal his first impressions of the 'Special One', admitting he has already been impressed with his new boss.
"He is much different than the image that many might have of him," he said.
"He is very calm and caring and he told me that he is thrilled that I am here and that he will help with me everything possible. We have spoken every single day; he asks my advice and I ask him questions. He puts me right in the thick of things and makes me feel important."