The Blaugrana shot-stopper was surprised the Spain No.1 was only named on the bench on Sunday.
Casillas had a hard time under former Blancos coach Jose Mourinho during the Portuguese's last season at the Santiago Bernabeu, which eventually resulted in the goalkeeper being benched for five months.
Lopez was brought in from Sevilla in January and was installed as the club's first choice keeper - a trend that continued on Sunday with Casillas left out of the side - and Valdes has admitted his astonishment at the decision.
"I was surprised that Casillas was a substitute," the 31-year-old told El Mundo Deportivo. "At the Confederations Cup he did quite well after a difficult personal situation and months without playing. I've seen Iker do the impossible, for me he is the best.
"I'm surprised by this decision, but it's not my club and I have not much to say about that. But he is the best. I would like to see him play more, it's a challenge for me to face him."
Barcelona is currently preparing for its trip to Atletico Madrid on Aug. 21, while Ancelotti's men will face Granada five days later.