Casillas, who was already under huge pressure following shaky recent form, made another high-profile error in Spain's 2-1 defeat to Slovakia on Thursday.
Del Bosque has continued to back his goalkeeper, but De Gea's performances for Manchester United could see the 23-year-old come into the XI for tonight's match.
"We'll see if De Gea or another goalkeeper plays. Whoever plays will deserve it. It's a difficult issue," Del Bosque told AS.
Casillas is also seeing his place at Real Madrid come under threat, with Keylor Navas knocking on the door.