Cristiano Ronaldo believes FIFA president Sepp Blatter was right to controversially describe him as being treated like a "modern-day slave" by current club Manchester United.

Ronaldo was responding to Blatter's comments made earlier in the day, in which the head of world football insisted the player should be allowed to join Spanish giants Real Madrid if he wants to.

"I completely agree with the FIFA president," said the player currently resting in Portugal after undergoing an ankle operation.

He added: "The president is correct, but I do not want to say more."

Real Madrid are believed to be prepared to offer around 85 million euros for the 23-year-old and pay him nine million euros on a five-year deal.

Ronaldo, a boyhood Real fan, has consistently been linked with a move to the Spanish capital since the end of last season after scoring an impressive 42 goals last term as United won both the Premier League and Champions League.

"There is no agreement with Real Madrid and I still don't know where I will be playing next season," Ronaldo said on Thursday, adding that he will be out for "10 to 12 weeks" to recover from his operation.

Wading into the protracted wrangle over the player's future, Blatter said that the current practice of tying players to long contracts amounts to "modern slavery".

And he urged Manchester United and Real Madrid "to sit together" if Ronaldo wants a move to the Spanish club.

"The important thing is we should also protect the player," Blatter told Sky News.

"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.

"I'm always in favour of protecting the player and if the player, he wants to leave, let him leave."

Blatter believes the issue raises questions about the way transfers and contracts are dealt with in the game.

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

"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."

United issued a swift rebuke to Blatter's outburst, insisting that all their players were happy to sign contracts with the club and knew the consequences of entering into such an agreement.

A United spokesperson said: "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's communications director William Gaillard, who serves as special advisor to European football governing body president Michel Platini, was also unimpressed by Blatter's outburst.

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

In contrast, Gaillard fears the Bosman ruling, which allows for free transfers at the end of contracts, has given players the upper hand and contributed to spiralling wages.

"It seems that both clubs and players are trying to negotiate an exit before the player is free," Gaillard said.

"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."

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