Los Blancos' captain was captured on camera during Tuesday night's clash with the Citizens, and appeared unmoved by the Portuguese's 90th-minute goal
Jose Mourinho's side were losing to the English visitors five minutes from time until Karim Benzema levelled matters before Ronaldo popped up to drive home the winner and send the home fans into raptures.
And in the pictures taken by TVE, Casillas appears indifferent as the Santiago Bernabeu erupted and the reasons remain unclear.
In recent weeks much speculation has surrounded the Spanish champions, stemming from Ronaldo's revelation of his apparent 'sadness'. The former Manchester United player's admission sparked off a media frenzy with question marks being raised regarding his relationship with certain team members, including Casillas.
Since then, however, Casillas has revealed that he was feeling sad during the game, due to the death of a young Polish football fan that he met during the summer. Marca suggests that this was the reason why he did not celebrate the goal.
The keeper expressed his grief on facebook, with a message stating: "My most sincere condolences to his family. Rest in peace, my friend".
Real Madrid take on Ajax in Amsterdam next on matchday 2 of the Champions League group stage.