The Germany international opted to leave the Santiago Bernabeu side for the Premier League this summer in a 50 million euro deal following the signings of Isco and Gareth Bale.
Courtois feels Madrid should have done more to hold on to the 24-year-old as he feels the club is missing an extra edge in attack after his departure.
"Without Ozil, Madrid lack a certain quality in their game," Courtois told AS. "Ozil had tremendous vision in playing the final pass between the lines and opening up the game, although it is true that they have others."
The Belgium international then went on to stress that last season's Copa del Rey final win over Real Madrid has helped ease the pressure off Atletico ahead of Saturday's derby at the Santiago Bernabeu.
"When I arrived at Atleti, all the talk was about the years we had not won a derby. Then came the cup final win, and that relieved the pressure in the sense that we no longer speak about everything that went on before," added the Chelsea loanee.
"Obviously there is pressure, but not as much. With Madrid, you not only want to win the three points but also to defeat a rival."