United's owing Glazer family has vowed not to sell its star winger to Real Madrid, but Blatter believes that Ronaldo should be allowed to leave.
"I completely agree with the statements of the president, he is right," Ronaldo told Portuguese broadcaster TVI.
Ronaldo reiterated his previously stated desire to join the Spanish champion despite still not submitting a transfer request.
"You know what I said, what I want and what I would like," he said. "Now I have to wait and see."
The 23-year-old Portuguese insisted he is still with the club he steered to triumphs in the Premier League and Champions League in May by scoring 42 goals.
"It is not true that I have agreement with Real Madrid," he said.
He said he did not know where he will begin the season, but he will be sidelined for 10 or 12 weeks while he recovers from surgery on his right ankle.
He said he would spend a further two weeks on crutches and it would be two months before he could kick a ball. 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.
"And we are trying now to intervene in such cases."
