United have reached a deal for Mata - Chelsea's player of the year for the last two seasons - subject to a medical and the agreement of personal terms, with the switch to be finalised once Mohamed Salah's move from Basle to Stamford Bridge is completed.
Mourinho insists he would have sold Mata to any of Chelsea's rivals, including Arsenal, whose manager Arsene Wenger was critical of the deal, if the move was right for the player and the club.
"A team can only play with 11 players, not 12," Mourinho said.
"Arsenal is full of top players, City is full of top players, and United is full of top players, so if Juan plays for them it means somebody else is not playing.
"So why not? Every club does this in other countries, so what's the problem to do it here?
"If a similar situation happens (and) instead of us, instead of Man United, it's Arsenal or Manchester City or Tottenham or Liverpool, we would do the same.
"If the offer was the same and Juan desired to go, it would be the same."
Chelsea twice had bids for Manchester United striker Wayne Rooney rejected in the summer.
Asked if the Mata move might earn Chelsea a favour in negotiations for Rooney, Mourinho said: "I don't think so. But I don't speak about Rooney. He's a Man United player, but I don't think so."
Spain playmaker Mata, who found opportunities limited this term, was permitted to move because all of the elements of the deal were right and because he requested to leave.
"It was a process, a difficult process for him, and not easy for me," Mourinho said.
"It's difficult for me to leave him out of the team. It's difficult for me to decide that Oscar is going to be my number 10 and that I'm going to build my team around that decision.
"And it's difficult for him to accept the situation. He's a player with status.
"He was behaving amazingly well, an example for the other boys to follow, but no happiness. With that status, it's difficult.
"Man United is obviously a big club, and when a player is not happy in this moment at Chelsea, but he loves to be in the Premier League and wants to stay in the Premier League and he thinks about the World Cup and about playing regularly every game, and he sees the stability our team has at the moment with Willian, with Oscar, with Eden (Hazard), he thought about leaving.
"He's tried to find the best option for him and he decided to go to United, which is understandable."