Mourinho hits back at Wenger
Wenger made the statement because Chelsea have already played United twice this season and will not have to face Mata this season, whereas other title contenders will.
The Spain international is close to joining United for a club-record £37 million ($61 million) fee.
Mourinho added: "Wenger complaining is normal, because he always does. It's something that we know.
"When a player plays for a club on August 20 and two weeks later on September 5 plays against the same club with a different shirt, this is normal, because this is the market.
"We don't do the market, we don't do the rules, we have just to do according to the rules.
"If Wenger sells (Mesut) Ozil to Man United in this moment I would be very happy because he sells a very important player.
"Normally he should be happy that Chelsea sold a player like Juan Mata, but this is a little bit his nature.
"When he says this is not fair, I think what is not fair is that his team always has the best days to play."
Mourinho and Chelsea moved quickly to replace Mata with Mohamed Salah after reaching an agreement with Basle, subject to personal terms and a medical.
"It was a surprise for us that Juan was leaving," Mourinho said.
"We were not prepared to make a bid or to try to bring a player to replace Juan in the squad so everything happened very, very fast.
"We decided to try to go for Salah. We started from zero; no contacts with the club, no contacts with the player.
"The first step was an agreement with the club. That situation is done, no more discussions, everything is signed and done subject to the medical, that we didn't have yet and subject to the player's desire to join us or not.
"Now we know he wants (to come)."
Mourinho has spoken to Salah, who was also a target for Liverpool.
Liverpool manager Brendan Rodgers has not given up hope and Mourinho admits a last-minute U-turn is feasible, although the Egypt winger's preferred destination is west London.
Mourinho said: "We know that the player wants to come. He told me personally.
"Now it's (personal) terms, the medical that he has to pass probably in Zurich or Lausanne and hopefully he comes.
"Probably the situation is still open and not closed."
Salah is set to become the third signing of the winter transfer window, following the captures of Bertrand Traore and Nemanja Matic.
Kevin De Bruyne has already left for Wolfsburg, Mata is en route to Manchester and Michael Essien has permission to leave, too.
"Michael Essien is in this moment in a position where he can go or he can stay," Mourinho said.
"If he stays we are more than happy for him to stay, but he's another player that gave everything to this club and if a great situation for him arrives for his future... we must show him respect.
"If he stays, he stays and if he stays he's playing against Stoke."
Chelsea play Stoke on Sunday in the FA Cup fourth round, with striker Fernando Torres (knee) missing for three weeks.