Spain coach Vicente del Bosque has used both De Gea and Casillas in the run up to the competition, though it is the former Real Madrid and current Porto shot stopper who has been relied upon most often.
The reigning European champions' preparations for the tournament have been marred due to De Gea's alleged involvement in an ongoing sex case which he has strongly denied.
However, Del Bosque defended the Manchester United No.1 shortly after the reports emerged and has since told the dressing room that it is the 25-year-old who will line up in goal for Monday's encounter.
The coach and Sergio Ramos have both declared that the mood in the Spain camp has not been soiled by the implications, but Del Bosque had said before the game that he was willing to wait to see if De Gea was in the right frame of mind before deciding to start him.
He said: "We will see if the player is able to play. If not, we will take another decision without any problem.
"We have to think about these situations for the team. We are not going to think about just the goalkeeper. We are going to think about the whole team."