David de Gea has admitted that he still has plenty of work to do to become Spain's first-choice goalkeeper, despite making his competitive debut on Sunday.
The Manchester United stopper replaced Casillas in the team after the Real Madrid man's error contributed to a surprise 2-1 defeat to Slovakia on Thursday.
De Gea kept a clean sheet as Spain ran out 4-0 winners against Luxembourg.
"I felt very comfortable with the team," the Manchester Evening News quotes De Gea as saying. "It's an important victory. Gradually you gain confidence and with the encouragement I get, it's easier.
"The coach decides who plays. I always come with all the enthusiasm I have to help the team. Iker has congratulated me."
De Gea has won three caps for Spain.
