The 25-year-old moved to Middlesbrough on loan in January, but had previously kept goal for and captained Castilla, Real's second-string side.
Since his arrival from Sevilla over a year ago, Lopez has played every La Liga game for the capital, while Casillas has been used for Copa del Rey and UEFA Champions League ties.
The rotation by Carlo Ancelotti has led to reports of tension between Lopez and Casillas, who has been a mainstay of the club for the last 15 years.
However, having trained regularly with the duo, Mejias denies that there is any problem at the club, insisting both men are ultimate professionals.
"I wish the best to him (Casillas) and Diego," he told AS.
"They are two great goalkeepers. We did not think about it (rotation). The two are performing and contributing to the team.
"That tension is positive, keeps them alert. You cannot ever relax.
"At Madrid, all are great professionals. It is the highest level. If someone is not fully trained it is impossible."
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