The former Real Madrid midfielder can see no other reason than a behind-the-scenes dispute to explain his former team-mate's continued absence from the starting XI
Diego Lopez has been the No.1 choice between the posts at the Santiago Bernabeu since joining the club from Sevilla in the January transfer window.
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"It's hard to judge from the outside," Granero 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 Rs midfielder did reserve praise for Lopez, however, and admitted that 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," he added. "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."