The Liverpool goalkeeper has insisted that his compatriot deserved better, and added that he wants the Real Madrid captain to return to fitness as soon as possible.
The Spain international lost his spot between the sticks to Antonio Adan for the matches against Malaga and Real Sociedad, but soon won his place back after his understudy was sent off against the Basques.
"I don't think relegating players to the bench motivates them. There are other ways of dealing with these things," Reina told Canal Movistar.
Casillas subsequently broke his thumb against Valencia, and Reina went on to say that he hoped his international teammate returns to full fitness as soon as possible.
"I have spoken with Iker and I sent him a text message of support when he got injured. I hope he is back with us as soon as possible.
"He has to recover soon and make sure he recovers fully. His hands are vital for this job and he must look after them."
Casillas is expected to be out of action for an estimated two more months due to his hand injury.
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