Lopez was signed in January following an injury to captain Iker Casillas and has retained his place in the starting lineup despite his Spanish compatriot's recovery.
"The coach decides who plays," ESPN quotes Mourinho as saying. "I do not flip a coin to see if Marcelo or Fabio [Coentrao] plays, or if [Luka] Modric or Xabi Alonso plays. I think about it, discuss it, study things for many hours and try to make the best decision.
"For me, I like Diego Lopez more as a keeper than Iker Casillas. It is not a personal decision. I like a keeper who plays well with his feet, who dominates his area and comes for crosses.
"Iker makes great saves between the posts, but I prefer a different type of keeper. As long as I am Real Madrid coach, Diego Lopez will always play."
Casillas has been at Madrid his entire career.