Arsenal manager Arsene Wenger has questioned the fairness of United's imminent move for Juan Mata.
The Spaniard is on the brink of a £37m switch to Old Trafford having been frozen out by Chelsea boss Jose Mourinho this season.
But Wenger believes the London club have not acted with integrity in choosing to let Mata go at this point of the campaign.
Chelsea have already played United twice in the Premier League this term and, by letting him go, have effectively unleashed him on their title rivals.
"It opens again a little bit the opportunity of this transfer market because Chelsea had already played twice against Man United so they don't play again," Wenger said.
"They could have sold him last week but it opens at least the opportunity [to question] the dates of this transfer window.
"Some teams have already played twice against one opponent and some others not. I think if you want to respect the fairness for everybody exactly the same, that should not happen.
"I just think you would want that everybody is on the same level. Chelsea do not make the rules but maybe the rules should be a bit more adapted to more fairness."
