LONDON, England (CNN) -- Cristiano Ronaldo should be allowed to leave European champions Manchester United for Real Madrid if he wants, says FIFA president Sepp Blatter.



Manchester United made a formal complaint to FIFA about Real Madrid's interest in Ronaldo last month.



Questioned about the protracted wrangle over the 23-year-old's future, Blatter told Sky News that the practice of binding players to lengthy contracts amounts to "modern slavery".



Ronaldo himself later agreed with Blatter's assessment of his transfer standoff with Manchester United.



United have vowed not to sell their star winger to Real Madrid, but Ronaldo has spoken of his desire to join the Spanish champions -- although he is yet to submit a transfer request.



He told Portuguese broadcaster TVI: "You know what I said, what I want and what I would like. Let's see, I agree completely with the president of FIFA. Now I have to wait and see, but I do not know where I will begin the season."



Ronaldo also added that he will be sidelined for 10 or 12 weeks while he recovers from Monday's surgery on his right ankle.



The Portuguese winger's future at Old Trafford has been the source of heated speculation, with Spanish giants Real making clear they want to sign him at any cost.



United made a formal complaint to FIFA about Madrid's interest last month, but football's governing body said that no regulations had been broken. "I'm always in favor of protecting the player, and if the player wants to leave, let him leave," said Blatter.



"If the player wants to play somewhere else, then a solution should be found 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."



Ronaldo, who hit 42 goals for United last season, has a contract until 2012 and Blatter believes the issue raises concerns about the way transfers and contracts are dealt with in the game.



He said he was "very in sympathy" with Ronaldo over his current position.



"I think in football there's too much modern slavery in transferring players or buying players here and there, and putting them somewhere," he added.



"We are trying now to intervene in such cases. The reaction to the Bosman law is to make long-lasting contacts in order to keep the players and then if he wants to leave, then there is only one solution, he has to pay his contract."



A United spokesperson told PA Sport: "All our players -- like at other clubs -- enter into their contracts after an open and free negotiation.



"Most of whom do after taking advice from a FIFA-registered agent. Many do so on a number of occasions and enjoy long and successful stays at Old Trafford."



UEFA have warned that players are becoming ever more powerful as a consequence of the Bosman ruling -- and played down Blatter's suggestions they are slaves to clubs.



Communications director William Gaillard, who also serves as special advisor to UEFA president Michel Platini, gave a different point of view and claimed salaries are "spiralling out of control".



"It would be useful to remind people that slaves in all of the slavery systems never earned a wage," Gaillard told Sky Sports News.



The Bosman ruling came into effect in 1995, allowing players free transfers at the end of contracts, and those coming to the conclusion of their deals have become prey for other clubs.



"It seems that both clubs and players are trying to negotiate an exit before the player is free," said Gaillard. "It is a consequence of the Bosman ruling -- there is nothing we can do about that.



"It is obvious that today players have a lot more power than they did 20 years ago, undoubtedly, and agents have a lot more power than they did 20 years ago. It is true that salaries are spiralling out of control. Many clubs have pointed that out."



Blatter went on to dismiss the Premier League's much-criticised
What was Ronaldo was later quoted as saying / What did Sepp Blatter say about long deals? Binding players to long deals amounts to what ? Who said Ronaldo should be alloed to leave Manchester Utd? Cristiano Ronaldo should be allowed to leavewhat team ? What should Ronaldo be allowed to leave?
I would like. Let's see, I agree completely with the president of FIFA. Now I have to wait and see, but I do not know where I will begin the season." "modern slavery". "modern slavery". FIFA president Sepp Blatter. European champions Manchester United
question: What was Ronaldo was later quoted as saying /, answer: I would like. Let's see, I agree completely with the president of FIFA. Now I have to wait and see, but I do not know where I will begin the season." | question: What did Sepp Blatter say about long deals?, answer: "modern slavery". | question: Binding players to long deals amounts to what ?, answer: "modern slavery". | question: Who said Ronaldo should be alloed to leave Manchester Utd?, answer: FIFA president Sepp Blatter. | question: Cristiano Ronaldo should be allowed to leavewhat team ?, answer: European champions Manchester | question: What should Ronaldo be allowed to leave?, answer: United