The former Real Madrid star and current assistant is pleased for the Spanish veteran, who has struggled to keep his place in the first team at the club level.
Casillas, who recorded his 150th cap against Finland on Friday, has found the going difficult at the Santiago Bernabeu since he broke his hand earlier this year and lost his place to Lopez in goal while under former coach Jose Mourinho.
New coach Carlo Ancelotti has continued the precedent set by Mourinho, with Lopez starting all Liga games in goal for Los Blancos, but, Zidane, a former Real Madrid great and current assistant coach, is pleased that Casillas' place as Spain No. 1 has not been affected.
"I am glad that Iker has held [his spot on the national team]. He is a great goalkeeper," Zidane said at a press conference for the Danone Nations Cup.
However, the former World Cup winner was giving nothing away on whether Casillas would be able to reclaim his spot in Ancelotti's side.
"We will see what happens at Madrid," Zidane said.
Casillas is in line to play against Chile for Spain on Tuesday before returning to club duties. Real Madrid's next match is against high-flying Villarreal and Mexican internationals Giovani Dos Santos and Javier Aquino on Saturday.