"I see a great improvement in the person, in the mood, in the face, in the body language", he said at United's Carrington training base.
The fact Burton view the early weeks of the league season as more important than a trip to Old Trafford highlights the issues with a competition that even last season's triumphant manager Mourinho believes could be dispensable. Four more goals take the club's tally to 23 in seven games across all competitions, with just three now conceded.
Herrera had famously said just last season - "I am playing now, I am getting a lot of minutes".
With a Champions League clash in Moscow next week, Mourinho could shuffle his pack with Marcus Rashford and Anthony Martial in the running to start after impressive displays in the midweek League Cup success over Burton.
"What I see? In a word; happiness". He's a happy guy, who's working well.
"If he starts matches, he tries to do well and if he goes on from the bench, even if it is for just 10 minutes, he tries to enjoy it and to give something extra in his time on the pitch", the tactician added.
But their run to the final last season contributed to a 64-game campaign during which they also reached the final of the Europa League.
Along with the goalscorers, Michael Carrick, Ander Herrera, Juan Mata, Daley Blind and Chris Smalling all started.
The United boss made his dissatisfaction clear both publicly and behind the scenes last term - tough love that appears to be having the desired effect with highly-talented Martial. "I think they look at things in very similar ways in that department".
The pair have been duelling it out for the left-wing spot following the arrival of centre-forward Romelu Lukaku and Rashford says they are driving each other on. "It's a process. He doesn't play for a long time". It's good news for us, because we need Virgil, like every single player in our squad.
City won 2-1 at West Brom in the third round on Wednesday, suffering an injury scare when Ilkay Gundogan, back in the starting line-up for the first time since December after a long-term right knee injury, hobbled off following a heavy challenge from Claudio Yacob.
"The next round, if somebody is better than us and they beat us... then okay, we're finished".
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