"I'm always in favour of protecting the player and if the player wants to leave let him leave," 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 think in football there's too much modern slavery in transferring players or buying players here and there, and putting them somewhere.
"We are trying now to intervene in such cases. The reaction to the Bosman law is to make long-lasting contracts 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."
However, these comments have riled United who have responded to Blatter's comments.
"All our players enter into their contracts after an open and free negotiation," a United spokesman has said.
"Many do so on a number of occasions and enjoy long and successful stays at Old Trafford."