Iker Casillas
Casillas at fault in Spain's defeat.Getty

Iker Casillas' role as Spain's number one choice in goal has once again come under severe scrutiny, possibly opening the door wider for Manchester United's David de Gea to step in.

Vicente del Bosque's team suffered their first loss in 37 qualifying games in a shock 2-1 defeat to Slovakia in their European Championship Group C clash on Thursday.

Casillas, who has remained La Roja's number one choice in goal despite an inauspicious World Cup, was deemed at fault for Slovakia's opener after misjudging Juraj Kucka's 30-yard free kick in the 17th minute.

While Valencia striker Paco Alcacer equalised for Spain in the final 10 minutes of the match they were unable to hold onto a point, with former Chelsea youngster Miroslav Stoch heading home from close range with three minutes remaining, another incident where Casillas' positioning was questioned.

De Gea, 23, had been expected to make his competitive debut this week after being called up to the senior squad for his country's failed World Cup bid and starting in the friendly defeat to France in September.

His superb performance during United's 2-1 victory over Everton last Sunday heightened calls for his inclusion in the starting XI but del Bosque has suggested he will not drop Casillas for the United shot-stopper despite the incumbent's most recent error.

"Casillas had saved a good chance from Slovakia in the previous move," del Bosque said to defend his captain. "I could not see clearly from the bench, but the ball seemed to move strangely in the air and Iker was deceived.

Meanwhile, Diego Costa's miserable run of form for his adopted country continued as his goalless run in international football extended to six games.

The 26-year-old, who has scored nine goals in his first seven appearances for Chelsea, had the opportunity to open the scoring after 15 minutes but headed wide from Cesc Fabregas' cross.