"It was a huge disappointment to go away from home, score two goals, and not get a win," says Glenn of the Charlton game.
"We have done it a few times this season and not to take even a point was criminal.
"Against Chelsea we had to score three to win; against Middlesbrough and Blackburn away we scored two and only got a point - and against Charlton we got nothing.
"At this level if you score two you expect wins and draws not defeats - it is not acceptable from the whole team but the majority of the responsibility does fall on the back four.
"What we have got to do is make sure we don't concede goals to turn wins into defeats; we need some clean sheets like you can't believe."
Thus far, efforts to bring in a defensive reinforcement haven't produced, but Glenn says:
"We are trying to bring another one in, but if we are not successful, the players we have got will form the team, and we will carry on working with what we have got until we find a successful formula."
Nigel Winterburn is again rested and Glenn explains:
"It is important that people realise Nigel Winterburn needed to be rested at Charlton though he was sub and ended playing 20 minutes, which was quite acceptable to him.
"He cannot be expected to play two games in three days and we have made the decision which game of Manchester United and Blackburn we are keeping him for.
"On top of that, at the first opportunity he needs a minor operation on his wrist; he will put that off until there is a break but he does need it before the end of the season."
With Glen Johnson likely to start after his midweek debut at the Valley, his boss adds:
"Our fans took to him straight away; they like to see what they would consider one of their own come through and he will always remember his debut - it is just a shame it was not better circumstances."
Of the trip to Old Trafford, he adds:
"There is no tougher place to go after losing 4-2 but the players have got to pick themselves up and remain positive -if we put in a spirited performance who knows what can happen?
"We will put out as strong a side as we can bearing in mind the injuries we have.
"At Charlton, we had a lot of long balls to deal with which meant it was very physical - we did get some bumps and bruises."
Joe Cole is still out with a calf injury and Glenn adds:
"Joe's calf needs to clear up before we play him again but Paolo and Fredi are looking sharper."