Blatter said the Red Devils should not hold the player against his will, even though he was under contract.
"I have never said that Ronaldo is a slave," Blatter told reporters on Wednesday during a visit to Vietnam.
"But I said slavery with young people, youngsters that European clubs are trying to get 13, 14 and 15 years old boys and bring them to the big clubs. This is in my expression, this is modern slavery.
"I have never mentioned that for Ronaldo.
"But the press, they have distorted it.
"Ronaldo is not a slave."
Blatter now hopes that the ongoing transfer saga surrounding Ronaldo will reach a conclusion which satisfies both United and Real.
"They will solve this problem," he said.
"We have nothing to say, Fifa is not going to intervene, let it be as it is now."