Wenger said: “I am [suprised] because Juan Mata is a great player.
“They are selling great player to a direct opponent.”
Chelsea have played United twice this season, meaning Mata won’t face his former club.
It’s prompted Wenger to call on the Premier League to look at the fixture schedule.
He added: “[It] opens up the opportunities in this transfer market because Chelsea have already played twice against Man Utd and so don’t play them again anymore.
“United could have sold him last week but it opens at least the dates of this transfer window.
“Some teams have already played twice against one opponent and some others not and I think if you want to respect the fairness for everybody – exactly the same – that should not happen.
“I wouldn’t go as far as that, [Chelsea selling Mata to do damage to rivals] I just think you would want that everybody is at the same level.
“I can understand completely what Chelsea are doing, it’s within the rules but maybe the rules should be adapted for fairness.”