The 24-year-old has dominated the Old Trafford news agenda this week after reports suggested the midfielder was irritated at playing so deep, and was even regretting rejoining United.
Mourinho was also reported to be annoyed by Pogba's inability to function in a two-man midfield, but all that talk has been dismissed out of hand by the United boss.
"I think you are nice with your words because when you say 'a lot of speculation' (regarding Pogba) you should say a lot of lies," the Portuguese said.
"Because I accept, and I can speak on Paul's behalf without any kind of problem, Paul accepts that he has not been playing well in the last few matches. But that's all.
"And if you want to speak about it, then that's one thing and then your word 'speculation' maybe makes a little bit of sense, but the majority of things you can read and listen, don't be nice, be objective and say (it's) lies.
"I don't have to speak with you about my conversations with my players. I don't have to tell you anything about it.
"It is my problem, it is the player's problem. A big lie that our relationship is not good, a big lie that we don't communicate, a big lie that we don't agree with his positions and his involvement in the dynamic of the team.
"So, be objective and say what we all know: in the last couple of matches he didn't play well. Period. End of story.
"Now, it's my problem and Paul's problem to deal with it and try to improve his performance level. You don't need to be a liar."
Mourinho confirmed Pogba will start at Huddersfield in an intense press conference previewing Saturday's FA Cup fifth-round clash.
Criticism and scrutiny from pundits has clearly got under the United manager's skin - "I would like them to tell me which position Pogba played in against Newcastle" - and threw questions to reporters in the room.
Mourinho clarified that Pogba's preferred formation is 4-3-3 and playing as an "eight on the left", before urging the media to explain the game and not talk "b******t".
"He had a big injury in the season," Mourinho said.
"He almost went to surgery like (Ousmane) Dembele in Barcelona went with a very similar injury.
"The decision was not to do it and go to a conservative treatment, which resulted very, very well. Everybody really happy with the way things went.
"He was playing phenomenal before that, he played phenomenal after that. He had a red card against Arsenal probably in his best match of the season, in his best moment and probably in the team's best moment.
"He was out for a long, long time and in this moment, he is not playing well. Period.
"And the team needs him at a good level and when he is not a good level the team is not as good as when he plays phenomenal.
"I think it happens with every team when the best players, the most crucial players for some reason are not performing. Period.
"You don't need to come with 'we are going to sell him', 'we want to sell him', 'he wants to leave', 'we fight every day', 'we don't see each other'. You don't need that, and some people go in that direction.
"Some of them I know them for many, many years, I know the profile. I know their surname is liar. There are people in your world that their surname is liar and that's not nice."
Eric Bailly is in line to return at Huddersfield after a three-month absence with an ankle injury, but Ander Herrera and Marcus Rashford will miss out with unspecified muscle complaints.
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