The 33-year-old spent large parts of the last two seasons playing second fiddle to Diego Lopez in La Liga, and a potential rustiness saw Casillas drop a rick in the UEFA Champions League before an error-strewn performance against Netherlands in Brazil.
Though the former mattered little as Real eventually topped city rivals Atletico 4-1 to seal a 10th European crown, the Dutch were far less forgiving, racking up a memorable 5-1 win in Salvador.
Spain eventually surrendered their world title meekly, going out in the group stage, but Casillas was back in action on Tuesday as Real won the UEFA Super Cup courtesy of a 2-0 win over Sevilla.
With Diego Lopez departing for Milan, Keylor Navas is Casillas' new rival for the number-one spot after a string of heroic outings for Costa Rica at the World Cup.
However, Ancelotti will not take the duo's close-season form into account when picking his team this season and has not ruled out Casillas becoming first choice again.
"What happened in the World Cup is completely forgotten about," the Italian said.
"It's the start of a new season. Will he [Casillas] be first choice? We'll see. Everyone has faith in him.
"There won't be a fixed starting XI, I will need to rotate the players and use all of them at some point."
Madrid's victory in the Super Cup on Tuesday was Casillas' 17th major trophy with the club since making his debut in the 1998-99 season.
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