The 22-year-old has been one of the biggest losers of Mourinho's arrival at Old Trafford in the close-season.
The 22-year-old, bought for £25 million ($30 million) from PSV Eindhoven by Louis van Gaal 18 months ago, has been one of the biggest losers of Mourinho's arrival at Old Trafford in the close-season.
And the United manager explained his decision to omit him from first team consideration sprung from Depay's wish to leave Manchester next month.
"I have to say that in the past couple of months my decisions in relation to Memphis were influenced by the feelings and information that he would like to leave in January and that we were going to have a real offer that we would be willing to accept," said Mourinho.
"That obviously influences me. If I know, if I have the feeling that a player is leaving -- if I have to give chances and develop other players -- then I go to Lingard, Mkhitaryan, Martial, the players I know 100 per cent who are going to stay with us.
"It is a position where we have more options."
Depay has been little more than a bit-part contributor for United this season and has played almost as many minutes for Holland -- 119 in two World Cup qualifiers -- as he has with his club.
For United, Depay has started just one game, a 55-minute appearance in a League Cup tie at Northampton, and been reduced to 20 minutes of Premier League football, spread over four matches.
In total, Depay has featured in eight games in all competitions under Mourinho, amassing barely two hours of football, 124 minutes.
"He was not playing last season," added Mourinho. "It was not that last season he was playing phenomenally and this season he is not playing at all. That's not the situation.
"He's a very good professional, is the only thing I can say. I know the public image is always influenced by some details of their private lives but the reality is that he is a very good professional, a very good boy.
"He has a high level of education, super polite, super professional, and works really hard.
"But it's a position where I have Martial, Mkhitaryan, Mata, Lingard, plus Rooney and Rashford who can also play in this position.
"So we have six, seven players. When I have the feeling and information that the player probably has a big chance to leave then I have to care more about the others.
"But if you arrive in January and nothing happens -- and we have lots of matches -- then if he is going to be my player until the end of the season I will help him and he will be useful for us."
Everton, desperate for cover for long-term injury victim Yannick Bolasie, remains Depay's most likely destination in the window, although Porto, Roma and AC Milan have also been linked with the player.
Meanwhile, Mourinho has admitted United fear losing 35-year-old veteran Zlatan Ibrahimovic to injury in the new year with no obvious goal-scoring successor ready to step into his boots.
The United manager has ruled out the possibility of signing a back-up in the January transfer window.
"No, we are not going to do that," said Mourinho. "We have Zlatan, Rooney, Rashford and Martial, who can also be a number nine, in a different style.
"We have a squad of 24 players. I know we've a lot of competitions to play but where is the striker to come here with that quality to play for United?
"Where is the club that can let him come? That's not something for the January market, it's probably something for the summer.
"But if we lost Zlatan? A disaster. Obviously a disaster but that's football."