Both players lined up for Liga champions Atletico Madrid during the last campaign, with Courtois spending a third year on loan at the club from Stamford Bridge.
With Petr Cech first choice for Mourinho, the future for Courtois looked uncertain given his reluctance to be back-up to the Czech international, however the Portuguese coach has given the clearest indication yet that he intends to use the 22-year-old.
"When the World Cup is over, Courtois will be coming straight back to Stamford Bridge," he told Spanish newspaper Marca. "End of story."
And Mourinho hopes to take another vital part of Diego Simeone's squad too after admitting that Costa is his top transfer target.
"Costa is a player I want in my team," he added. "I've asked the club to bring him in.
"I am extremely confident that we are going to get him, and he'll end up signing for Chelsea."
