Glenn casts an envious eye back to his opponents' bench at Old Trafford on Saturday and says:
"When David Beckham is fit the England manager always picks him but Manchester United were doing so well Sir Alex decided he didn't want to change a winning team - and had the luxury of being able to leave England's captain on the bench, which must be fantastic.
"And it isn't half an incentive for the players on the pitch to carry on playing well.
"There is no doubt about it, I am crystal clear on this, the best motivation to keep players really at it and on top of their form is pressure on their place.
"It is human nature; if players know they can have a poor game and not be left out that is not a great incentive to play well."
With injuries - and, in the case of Eduoard Cisse, illness - adding to Glenn's problems, options are limited and he says:
"When you have a small squad like in our case it is so important that the players in it are self-motivated, players that want to play well whatever the situation.
"That is really key and critical - that you don't have the sort of players that needs pressure on their place to play well.
"At Manchester United there are so many top international players waiting in the wings for an opportunity to play.
"They are coming to the boil at the moment and I am sure Sir Alex must be happy with the performances he is getting from his team.
"We have played excellent football but haven't got the breaks we deserve; I have seen teams play nowhere near as well as we have and winning.
"We just need to have a bit more of a killer instinct in front of goal and be that little bit tighter at the back so our good play can allow us to win games."