The 21-year-old Belgian has been on loan at Real's fierce rivals Atletico since 2011, as Petr Cech continues to block his path to the No.1 spot at Stamford Bridge.
But, as some media outlets suggest Courtois could be set for a permanent switch to the Spanish capital, Mourinho insists Courtois remains a Chelsea player for the time being.
"We don't struggle to keep hold of [Courtois]," the Blues boss told reporters. "I don't need to give assurances to any player.
"If somebody wants him, we have to know. It is easy for us.
"When you want a player and the player has two more years of contract. It is easy to say that we have no problems."
Courtois has made 134 appearances in all competitions for Atleti since first arriving on loan.