The stirring show put on against Liverpool was followed by a two-week gap due to Cup final and the level of performance was not repeated at Filbert Street.
"There are no excuses for that performance," insisted Glenn. "We were not playing at our best up to half-time even.
"They were controlling periods of the game, but once you go 1-0 up with 25 minutes to go there is no way that should have happened.
"We didn't play well really from the moment we went ahead, which is an incredible indictment to the team. It was a dreadful performance really."
Glenn says the defeat has given him plenty to mull over during the summer months.
"In many ways it's going to give me food for thought over the summer, things have got to be changed. I will change things and it just shows you that there is a lot of hard work to be done.
"At times we've played some smashing football, but at times in this league you've got to dig in deep and show character - we didn't do that.
"We wanted to finished seventh, which with reaching a final and a quarter-final would have been a half-decent season. A good foundation to build on.
"Quite frankly, this result, and the way we lost it from 1-0 up, shows the work that's got to be put in."
Glenn Hoddle said the gap between fixtures was no excuse for the performance shown against Leicester on Saturday.