Del Bosque had started Casillas against Finland last month and only given Valdes the nod in a critical 1-0 at France in March because Casillas was injured.
But Del Bosque said that Casillas' recent lack of playing and training time had influenced his decision to go with Valdes, who is having perhaps the best season of his career.
"These were different situations," Del Bosque said. "For Finland, I had all my goalkeepers arriving at the same level. But for this game Iker had gotten hurt at Galatasaray and had only practically played one game in the last month."
Del Bosque would not say who would start Tuesday's qualifier againt Georgia when Spain will be playing to clinch its place in Brazil.
In another surprising move, Del Bosque started Swansea forward Miguel "Michu" Perez in his first international game.