"It is a fine line; Leicester have had three or four good seasons, but finished the season poorly last year and had one win in about 10 games," he says.
"They started this season poorly, and now they have been relegated.
"Ipswich is another classic case - they had a brilliant season last year and surprised so many people, and now look what has happened.
"They are not down yet and quite honestly I hope Ipswich get out of it because I like the club and the people there and they play attractive football.
"But they are in the mire, and that's how easily it can slip, and the team we are playing tomorrow also falls into that category."
For Sunderland too, after two top eight finishes in a row, they now find themselves still looking to mathematically avoid relegation.
"They have had a couple of great seasons; Peter Reid has done a marvellous job and it is so unfair the criticism he has had this year considering what he has done for that club," adds Glenn.
"With them so close to 40 points they will be really fighting; they certainly gave Liverpool a game last week, and we know we have got to have our eye on the ball, concentrate hard, match them all the way, and try to play the way we always try to play."
As for the draw at Tottenham, he says:
"It is very important we got that late goal to remain unbeaten in four games and just as important to keep Tottenham below us.
"If they had taken all three points I think it would have been undeserved, but we have got ourselves into seventh and we want to stay there.
"The difference was, which hasn't always happened this season away, is that although we lost a goal we didn't lose a second goal quickly - or at all.
"That effectively killed the game and while it is 1-0 there is always a chance, which is how it panned out for us.
"At times this year we have been so gung-ho away it has cost us embarrassing defeats, but it is never too late to try and put things right, and that's what we are working on in our last two away games, albeit that they are very hard ones.
"But we are focused on one game at a time, and the next one is against Sunderland who are fighting for their lives, and are going to come and scrap for everything.
"They will try and stop us from playing but we have got to meet them head on and carry on the unbeaten run from four games to five.
"We have got to be fair to ourselves, that is the way I see it; we have got no favourites but our own team and we have to be ultra-professional and not let ourselves or our fans down.
"We want to put on a good show and keep the attitude going as we have done since we got to 40 points; there are teams who have eased off when they got there and suddenly they are getting beaten two or three times.
"We don't want to allow that to happen - we don't even want to think about the summer break until the final whistle is blown on May 11th against Bolton."