Prosecutors have filed a complaint in which they accused Ronaldo of a "voluntary" and "conscious" breach of his tax obligations, amounting to a figure of €14.7 million.
The Ballon d'Or holder's representatives Gestifute have denied the accusation, while his club Real Madrid have said they are "absolutely convinced that our player Cristiano Ronaldo will prove his total innocence in this process".
Former FIFA president Blatter is serving a six-year ban from all football-related activity over a "disloyal payment" made to former UEFA chief Michel Platini.
In an interview with RTS, the 81-year-old Swiss claimed Ronaldo's situation is a consequence of endemic distrust in football compared to other elite sports.
"One would never suspect [tennis great Roger] Federer, a money-making machine," he said.
"Federer is untouchable."
Reflecting on the temptation for athletes to break the rules on the field, Blatter cited the example of Diego Maradona's infamous 'Hand of God' goal against England at the 1986 World Cup.
"It's not good, but we cheat anyway," he said.
"That's why this sport [football] has set itself some safeguards."