The Portugal Football Federation confirmed the Real Madrid star was suffering with a "muscle injury on the back of his left thigh as well as left-leg patellar tendonitis" on Tuesday.
Ronaldo's absence at the competition would represent a major blow to Portugal's chances of reaching the latter stages but Nani says the FIFA Ballon d'Or winner is "relaxed".
The Manchester United man also sought to play down the impact Ronaldo's potential absence would have on their hopes in Brazil.
"We hope that Cristiano is in good enough shape to play - and to help the team," he said.
"(Ronaldo) is going well, and he is relaxed about it... things are going the right way.
"If for some reason (Ronaldo) is not there, whoever is there playing has to be confident in their ability to get the job done.
"The World Cup is a one-of-a-kind challenge. I want to be fully fit, and in top form alongside my team-mates. I could not be there last time, but happily, I am here now."
A succesful World Cup in Brazil would cap off another impressive season for Ronaldo, who was crowned the world's best player in January before winning the Copa del Rey and the UEFA Champions League with Real.
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