The 28-year-old has had a fair share of highs and lows since last year's big-money switch from Borussia Dortmund, with the attacking midfielder currently going through a challenging period.
Last season he recovered from an inauspicious start to play a key role in United's Europa League triumph, but this term a promising start to the campaign has markedly tapered off.
That drop-off led Mourinho to omit him from the last two matchday squads and the Portuguese wants a reaction as he prepares to bring the Armenian back into the fold in Saturday's Premier League clash against Brighton.
"I was not happy with his last performances," the United boss said. "I'm not speaking about one or two, I'm speaking about three, four or five.
"He started the season very well and after that step by step he was disappearing.
"His performance levels in terms of goal scoring and assists, high pressing, recovering the ball high up the pitch, bringing the team with him as a number 10, were decreasing step by step.
"That was enough because the others worked to have a chance. Everybody works to have a chance. It's as simple as that."
Hard work on the training ground can only go so far, though, as it is not the "best habitat to analyse the players" when there are so many games.
"I would say on the pitch – pitch, match, and sometimes in a couple of minutes you see things," Mourinho said.
"You don't need sometimes to play 40, 50, 60 minutes for the coach to have feelings and have data to analyse.
"So I would say it's on the pitch, especially on the pitch. Tell me on the pitch what you want to do the next match."
Mourinho certainly will not shy from telling his players what to do, especially after slipping back into old habits as a lack of cutting edge saw them lose a match in Basel they dominated.
The 1-0 midweek loss does not seriously hamper their chances of Champions League progress, but underlines why his exciting attacking options cannot be considered the best he has had.
"No, because a team that plays the way we play so many times, I think we should score more goals than we do," he said.
"The team that excites more is the team that after five chances is winning 4-0. That's a team that excites more.
"I think they give what they can. I cannot complain about the dedication or the motivation. They all try to give their best.
"I think we had an improvement with (Anthony) Martial. He's scoring more goals than last season. That's why he is playing more. He has more minutes on the pitch.
"The other players are what they are. They have different kinds of qualities.
"Sometimes they produce a lot, and we have lots of chances and we don't score enough.
"The reality is that in the Premier League by not scoring enough we are still, I think, the second or third team with the most goals scored. So not dramatic."