Lopez will deputise for the injured Iker Casillas tomorrow at the Bernabeu and Wigan boss Martinez says United would be wrong to view him as a weak link.
"He's one of those goalkeepers, who you would say is not outstanding at just one thing and he does not have any weaknesses," the Spaniard told the Mirror.
"For a goalkeeper, that is a real strength.
"His distribution is at the same level as Pepe Reina and everything else is good, or even very good.
"He is good in one-v-ones, he covers good angles, he is patient and he doesn't commit himself. He hasn't had any red cards in his career.
"He is good in the air, controls the tempo well. He is a very good, experienced goalkeeper."