Barca and River go head to head at Yokohama's International Stadium on Sunday having seen off Guangzhou Evergrande and Sanfrecce Hiroshima respectively in the last four.
Champions League winners Barca are heavy favourites to win the global title for a record third time, but Iniesta has urged caution.
"I don't like to talk about favourites in a final. It's 50-50," he told reporters on Saturday.
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"River worry [us]. They are a strong team and they have quality players in midfield and in attack."
Barca star Lionel Messi starts the final after missing the 3-0 win against Guangzhou as a result of renal colic, and Iniesta had been hopeful the Argentinian would feature in the final.
"It seems he is better," he added. "Everyone wants him to play."
A triumph on Sunday would see the Catalans clinch their fifth trophy of 2015.