Injuries to Marcelo and Fabio Coentrao forced Mourinho to play Raphael Varane in the full-back spot in the 2-1 Champions League defeat to Borussia Dortmund on Wednesday.
Los Blancos have also been linked with moves for Chelsea's Ashley Cole and Tottenham Hotspur's Gareth Bale, but Mourinho responded sarcastically to questions about what he plans to do about the position.
"Tomorrow [against Mallorca] the left-back will be Higuain," joked Mourinho.
"He is fast, strong, he crosses well with his left foot and he scores goals."
The former Chelsea manager was more serious when discussing the rest of his defence.
"Ramos is alright and can play. He is a player who thinks of the team and has made himself available because he knows that the team has problems at the back.
"He and Pepe are very good, while [Ricardo] Carvalho is working well."
Real are currently fifth in La Liga with 14 points from eight games.