Courtois signed for Chelsea back in 2011 from Racing Genk - but he was immediately loaned to Atletico Madrid, where he has stayed ever since.
Courtois has emerged as one of the world's best goalkeepers, which has led to the likes of Real Madrid and Barcelona being linked.
Courtois's father has insisted talks will soon take place with Chelsea to determine his son's future - but Mourinho believes that he will be under his guidance next term.
"I believe so, I don't think he will be loaned out," Mourinho revealed.
"We don't struggle to keep him because he has two more years on his contract. I don't need to give any assurances to any player.
"A player that has a contract with Chelsea has a contract with Chelsea. Chelsea has Petr Cech, Courtois and Schwarzer. We are in a good condition."