Chelsea have sanctioned the mid-season sale of Mata - the club's player of the year in each of the past two years - despite the fact the deal will significantly strengthen a Premier League rival.
The Spain midfielder cannot face Chelsea this season - Jose Mourinho's side have played United twice already - but could yet affect the results of the club's title rivals.
Wenger takes issue with the rules which allow such unevenness but believes the deal is less down to astute timing and more do to with balancing the books to stay in line with UEFA rules.
"I think it is a purely financial reason," he said. "That is the first time I come to that conclusion with Chelsea.
"It means somewhere that Chelsea changes philosophy and they want to get on line with financial fair play. For me that is the main reason, and that is a good thing.
"We didn't need the rules of financial fair play to be dictated to us because we imposed it on ourselves, like what should be the case for everybody.
"It is difficult because you fight with a gun against people with a tank. Today, everybody now fights on more even ground.
"Chelsea sold Mata because they had plenty of players who are very young - Eden Hazard, Oscar, Ramires, David Luiz, they are young. They are not in a big need to buy.
"They had a surplus with Mata, so they sold him to Manchester United - yes it is a surprise, but they got a good price."
Kevin de Bruyne has also left Stamford Bridge this month, while Mohamed Salah and Nemanja Matic have come in.
Thecat...Don't resort to their level with tit for tat, unfortunately despite the brilliant business but terrible owners they have United are in a position where they can compete regardless, they turnover over 100 million more and have a lower wage bill, minus 20 million on interest, money they will start paying in dividends even taking off 40 million for no Europe, they can still compete and pay Rooney the most amount of money anyone is paid in the Premier League (and he still wants out??)...The fact Synergy misses the huge difference between what we are doing and PSG, he also completely ignores that Chelsea are slowly becoming self-sufficient (they even voted in favour for FFP in the Premier League)...He says it is not hard to buy a title, well lets hope for his sake it is not hard to try and buy 4th place, as that is what some club in Trafford is trying to do.- whyalwaysme