De Gea is expected to be given the nod to start against Georgia in Tuesday's international friendly, but Casillas played against South Korea last time out as the pair go head to head for the gloves.
Despite the European Championships beginning this weekend, Del Bosque is still undecided on the goalkeeping spot in his team.
"It's possible that De Gea will play tomorrow - Casillas did so against South Korea and that's all we can say. I don't have to make that decision now, and I'll make my mind up at a later date," Del Bosque told Marca.
Sevilla goalkeeper Sergio Rico will travel to France as Spain's number three.