Jose Mourinho says a "big injury" earlier in the season is not the reason for a dramatic loss of form suffered by Manchester United's star midfielder Paul Pogba.

The France international, United's record signing, was substituted in the second half of a 1-0 Premier League loss at Newcastle United on Sunday.

Pogba had been dropped for United's previous game, a 2-0 win at home to Huddersfield, having been off-the-pace and again substituted in a defeat at Tottenham at the end of February.

Mourinho hit out at media reports Pogba was unhappy at Old Trafford, decrying the speculation as "lies" in a fiery news conference on Friday.

Although Pogba missed two months with a hamstring problem earlier in the season, Mourinho denied lingering injury issues are to blame for the midfielder's struggles.

"He had a big injury in the season, a big injury," Mourinho told reporters.

"He had a big injury, almost went to surgery like [Ousmane] Dembele at Barcelona, the decision was not to do it and have conservative treatment, good results, everyone very happy.

"He was playing phenomenal before and after that, he had a red card against Arsenal in his best match [for United], in his best moment and the team's best moment.

"He was out for a long, long time but in this moment is not playing well, period. Period."

United have lost their last two Premier League away games without scoring a goal, which Mourinho indicated is partly due to Pogba's below-par displays.

"The team needs him at that level and when he is not, the team is not as good as when he plays phenomenal," Mourinho added.

"It happens at every team when some players are not performing, no need for histories or saying he wants to leave or we are going to sell, we don't need him, some people [pundits] go in that direction.

"I know their profile and their surname is liar - people in your [the media] world, their surname is liar - and that's not nice.

"Enough of Paul, he plays tomorrow."

Mourinho's men travel to Huddersfield for a fifth-round FA Cup tie on Saturday, having lost at the John Smith's Stadium on their Premier League visit in October, and the Portuguese is expecting a challenge against David Wagner's side.

"In the FA Cup last match [against Yeovil Town] we went to a really serious difficult stadium, opponent living a dream to play Manchester United and be a giant-killer, and we did it serious," Mourinho said.

"Obviously Huddersfield are better than Yeovil with different ambitions. I think Yeovil wanted to be a giant-killer, Huddersfield really want to progress and try to make something beautiful in the Cup, so we have to be ready. 

"We played Huddersfield very recently, a couple of weeks ago, we know who they are and how they play, we lost in their stadium so we can't be caught by surprise."