"I completely agree with the statements of the president, he is right," Ronaldo told Portuguese broadcaster TVI.
When Blatter was asked in a Sky News interview if he felt sympathy for Ronaldo's current position, he said: "Absolutely, absolutely."
He then urged United to sell Ronaldo if he asks to leave.
"We should also protect the player and if the player wants to play somewhere else, then a solution should be found," Blatter said.
"Because if he stays in a club where he does not feel comfortable to play then it's not good for the player and for the club . . . I'm always in favor to protect the player and if the player, he wants to leave, let him leave."
Blatter suggested that United was unfairly retaining Ronaldo.
"I think in football there is too much modern slavery, transferring players or buying players here and there, and putting them somewhere," he said.