Manchester City manager Manuel Pellegrini does not believe transfers such as Juan Mata's impending switch to Manchester United should be allowed mid-season.
Spanish playmaker Mata appears set to join United in a club-record move from Chelsea.
Pellegrini has no issue with United strengthening their squad during this month's transfer window - as they are entitled to do - but he is against the principle of big-money moves during the campaign.
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The Chilean said: "I think Juan Mata is a very good player.
"It is a shame he didn't play often this year at Chelsea. I think United are buying a very good player.
"Personally I don't agree that the player can go from one team to another team in the same league at this part of the year, but rules are rules and he can change clubs."
When asked why he did not agree, Pellegrini, speaking at a press conference, said: "Because I think if a player plays for a club for more than half of the season, a club with money can take the best players from the other teams.
"But the rules allow it, so (there is) nothing to say."
Arsenal manager Arsene Wenger believes this move highlights how the January transfer window is an unfair concept.
There is a feeling that Chelsea will doubly benefit because Mata cannot play against them this season.
Chelsea have already played United twice this term while he would be ineligible were they to meet in the Champions League.
United do, however, still have to play some of Chelsea's title rivals in City, Arsenal and Liverpool.
But Pellegrini said: "I am not talking about the reasons of Arsene Wenger. I am not talking about that.
"Maybe Manchester United have an advantage because Chelsea wanted to sell him. Maybe Manchester United have an advantage in this case.
"I am talking about general reasons and not the specific reason that they have played already against them.
"I think in this transfer window, players that play more than five or six games for the same club cannot change to another."