The Middlesbrough boss has reflected on his time at Madrid and insisted the goalkeeper's demotion had nothing to do with his relationship with Jose Mourinho
The Spain international, 32, lost his starting berth to Diego Lopez in 2012-13, with his supposedly poor relationship with Mourinho cited as one of the reasons for his demotion to the bench.
However, Karanka has now stressed that Casillas was dropped from the starting XI because they were unhappy with his displays, while admitting that his relationship with the goalkeeper has deteriorated as a result of last season's events.
"We thought a lot about his situation because we knew he was not just any player. We thought he should be performing better," he told La Sexta.
"I don’t know if Iker understood why he was benched. The same thing happened with [Fernando] Hierro, [Manuel] Sanchis and Raul. I think that he must have understood the situation.
"We were friends. We don’t have that relationship now because I left the club, and in all honesty, during those three years, especially towards the end, our relationship wasn’t great."
Karanka left Madrid at the end of the 2012-13 campaign and is currently in charge of Middlesbrough.