"The longer I am in this job the more I realise the importance of that one word 'attitude' - it is everything," he says.
"I like reading football books and Rinus Michels, who is the Godfather of Dutch football, made what to me is his most famous quote, which was: 'Give me good attitude and I will give you a good training session'.
"What he is saying is that if he has players with a poor attitude to work then no matter how good a coach he is, even he can't produce a good training session."
Glenn believes Lee Bowyer is one of those who puts as much into training as playing, and adds: "He very much falls into what I have just said. I am learning more about Lee every day.
"He trains hard, he enjoys it, he doesn't stop running in training, and he trains how he plays.
"That is another saying, 'you get what you train' - you can't possibly expect to train poorly all week and then play well on a Saturday.
"Good form is not a light switch that you just turn on and off; if you train well all week there is a much better chance that you are going to play well whenever you play.
"The ones that you can affect the most are the young players, and that is why I am always on about attitude to people like Jermain, who does train well, Joe, whose training has improved so much over the last year, Michael, who has pushed himself hard in training this year, and we are now on to the Glen Johnsons and Elliot Wards.
"I will not accept a sloppy training session off any of those young players and I don't accept it from any of the senior players, but sometimes you have to say, to only one or two, that their attitude is not acceptable.
"But the young boys know they won't get to the end of a sloppy training session before they are pulled up and told.
"I just hope that the penny drops and that habit goes into their senior football careers because if you have good habits when you are young you will have good habits when you are old.
"That is all part of the learning curve."
Glenn admits that not everyone's attitude at Old Trafford on Sunday was spot on, but says such a criticism is not one he has been able to make too often this campaign.
"It is the first time I could accuse a number of players of not fighting all through the 90 minutes.
"The players did not fight as hard as you'd have wanted but it is wrong to say all 11 didn't; one or two were okay."
Now, with a return to the league looming on Wednesday, he adds: "Last year we got through mainly on home form but that has been awful this year.
"We generally had the same pairing at the back in Tomas Repka and Christian Dailly; Tomas, who is back from suspension on Wednesday, did particularly well last season.
"He is a quality defender otherwise he wouldn't have been asked to go to Serie A and played 104 games for Fiorentina, not to mention playing in the Champions' League for two of the three seasons he was there."
Glenn insists he can recover - and make the players do so as well.
"I have to pick myself up - my own psychology will do that for me, and I am capable of doing that."