So outstanding for Borussia Dortmund in recent seasons, expectations were naturally high when the 27-year-old pitched up at Old Trafford in a £26.3million deal.
Mkhitaryan, the third signing of the summer, penned a four-year deal with the option to extend for a fifth, but after just a matter of months there are questions over the Armenian's future.
While a thigh injury played a part in restricting him to just 104 minutes of Premier League football, his return to fitness has not brought with it a recall to the squad.
Mkhitaryan's omission from Wednesday's EFL Cup win against Manchester City - the last team he lined-up against back on September 10 - fuelled speculation about his position, but Mourinho insists it is merely a matter of sharpness keeping him out.
"He's not injured," the United boss said
"Sometimes I confuse the meaning 'fit' in the English language.
"'Fit' sometimes you have the view is if you are not injured and sometimes 'fit' is in great condition and ready to compete
That's a different thing for me.
"In Portuguese, we use different words for the different situations.
"Mkhitaryan is not injured, he is training with the team 100 per cent.
"Some players, their profile they are really adapted to come and to play.
"Some other players need more time - time to feel it, to feel the intensity and the aggression, the game without the ball, the competitiveness.
"Many times players come from different countries
The realities are different in the style of football is different and the realities are different, for sure, in terms of competitiveness.
"There is no other country where it doesn't matter the team you play against, you have to play at the highest level because if you don't you have no chance.
"Does Mkhi needs time to become the top player we know he can be? Yes, I think he needs."
It is, Mourinho insists, just case of Mkhitaryan getting used to the English game rather than anything deeper, pointing to well-known examples at his previous clubs.
"I think it works, for sure," he said of the Armenian
"I don't like to speak about players from other clubs but I don't think they will be upset with me.
"Willian, when he arrived at Chelsea, had problems to play
He went through a process and then after a few months he became a top player, then, the next season when the team was champions, was absolutely phenomenal and still is.
"When Angel di Maria arrived at Real Madrid coming from Portugal, he also had to learn a lot about his game adapted to a new reality.
"Sometimes it happens
It happens sometimes with defenders but I think for defenders it's easier to adapt.
"For the attacking players, sometimes they arrive and go straight to success
But some other times it takes a little more time.
"In Mkhi's case, the process was interrupted by an injury that kept him out for about a month.
"When you are in this process of evolution, to be out for a month and then come fit but not fit to play, you have to go through a process of getting match fitness
Then when you have that, you need competitive level to do it.
"Of course we believe in him and of course we know that sooner or later (there will be) no problem."
Whether Mkhitaryan is called upon against Burnley remains to be seen as United look to build on the important 1-0 midweek cup win against City.
It felt like a crucial victory following last weekend's 4-0 humiliation at Chelsea, with the arrival of Sean Dyche's side offering the chance to get the Premier League campaign back on track.
"After the defeat, we had to play Man City two days after and because of that defeat, the match became more than just a normal cup match," Mourinho said.
"It was more because it was City but it was much more because two days before we had such a bad defeat.
"Sometimes people react to bad defeats in a negative way and you go into a bad run of results.
"But in this case, the boys managed to find that extra effort and extra competitive mentality to play that game because it means more than just a game.
"It was very important for the players to win it and to give back to the fans."
Source : PA
Source: PA