The former Blancos midfielder can see no other reason than a dispute behind his former team-mate's continued absence from the starting XI
Diego Lopez has been the number one choice between the posts at the Santiago Bernabeu since joining the club from Sevilla in the January transfer window.
And though Mourinho has remained insistent that his preference for Lopez is purely a footballing decision, there remains a suspicion that there may be more to the tale than meets the eye.
“It’s hard to judge from the outside,” he told AS. “Obviously there is a conflict. I know Iker very well and he is the best goalkeeper in the world. He is the captain of the team and has lifted the World Cup and the European Championship twice.
“He is a symbol of Real Madrid. He has spent 20 years at the club and to me has always been an example.”
The QPR midfielder did reserve praise for Lopez, however, and admitted his performances in recent months have warranted a first team spot.
“I am not the coach who evaluates the status of each player. You can say Diego Lopez has done nothing to not deserve this spot. I have seen him train like crazy in his life. He is very professional.
“But Casillas is Casillas and he is the undisputed starter. The coach is smart enough and professional enough to choose on form.”