Spain begin their tournament campaign against their neighbours on June 15 in Sochi and team captain Iniesta is wary of facing the Euro 2016 champions.
Iniesta has faced Ronaldo many times in El Clasico, knowing exactly what the five-time Ballon d'Or winner has to offer for club and country.
The midfielder also urged his team-mates to be fully focused in all their games as he fears the matches will be tight affairs, particularly the opener with Portugal.
"Portugal is a team that takes a lot to defeat and we will have to perform at the highest level, in a World Cup any small thing can knock you out," Iniesta told Cadena Cope .
"Cristiano is a fundamental player for any team he is in and for Portugal as well."
Iniesta called time on his Barcelona career this summer, leaving Camp Nou for Vissel Kobe after 16 seasons at the club. Despite turning 34 in May, he has not yet fully decided if he will hang up his boots on international football after this summer's World Cup.
However, he admits that it may be the right time to finish playing international football, especially as he will be playing in Japan from next season.
"I will never close one hundred percent the door of the national team after the World Cup, but everything indicates that this will be the case," he admitted.
During his time at Barcelona, Iniesta won 32 trophies, including four Champions Leagues and nine La Liga titles, playing 674 times for the Blaugrana.
He admits that leaving the club was very difficult for him, but he has enjoyed his last moments at Barcelona. Iniesta also believes that Barca president Josep Maria Bartemou hopes he can finish the season with another trophy - the World Cup - although thinks that if Argentina and Lionel Messi win instead, the club chief will still be pleased.
"These weeks of farewell have been very intense, emotionally very powerful, and I have tasted them to the fullest," he said.
"I understand that Bartomeu wants me to win with a national team that contains many Barca players. I suppose if Spain wins he will be happy, but if Argentina wins he will be happy too."