Wenger says Mata deal not fair
It is understood that the Spaniard will move to Old Trafford, with reports suggesting that the Blues have accepted a £40m offer from the Premier League champions.
However, Wenger believes it is unfair that Chelsea are selling Mata to United, particularly as the two clubs have played each other twice in the Premier League this season.
"I am surprised. Juan Mata is a great player and they sell a great player to a direct opponent," Wenger said.
"Chelsea have already played twice against Man United, they could have sold him last week.
"I think if you want to respect the fairness for everybody, this should not happen.
"I can understand completely what Chelsea are doing, and they do not make the rules, but maybe the rules should be a bit more adapted for fairness."
Arsenal, who are two points ahead of third-placed Chelsea, could face Mata when United travel to the Emirates on February 12.