The boss admitted that he did not see the turn-around coming and that he was very disappointed that the team 'switched off'.
"I didn't see where that second half performance was coming from, to be honest," said Glenn afterwards. "At half time I felt we were probably worth more than a one goal lead.
"We nullified everything they had attacking wise - I can't remember them creating anything in the first half.
"Quite frankly, the second half shows what happens to you in this league if you switch off. I didn't think our work rate was there like it has been, the squad's a bit stretched at the moment and we looked a bit fatigued I must say.
"But in saying that, I didn't think the attitude and work rate was there and consequently we get punished.
"They've got good players on the pitch, we knew that from the start. The likes of Shearer and Bellamy, you can't give them opportunities like we did in the second half and hope to win the game. So they punished us when they got the chances.
"I didn't see it coming. At half-time we were buoyant and I felt we could go out there and get the key goal.
"We had some great opportunities, Steffen Iversen had a great chance, Deano had a couple of chances and looking back, they were the key moments for us when we could have won the game.
"But I didn't like the way we played in the second half - at all.
"No desire to win the ball back, we were stretched as a team, all the things we haven't been doing this season and we got punished."
Glenn dismissed the theory that the team went into cruise control after securing the lead.
"I don't think it was a case of coasting - I've seen us play better and I said that at half-time. We were doing enough to be two-up, but at one-nil you're never safe in the Premiership.
"We may have been a bit leggy, a bit tired. They took a chance second half and played two v two at the back a lot of times, but we didn't look sharp enough to really capitalise on that. That was disappointing as well."
Glenn was also unhappy that one of the smallest players on the pitch, Acuna, was allowed a free-header for Newcastle's leveller.
"It was poor marking, it really was, and that's something we've been quite good at from free-kicks. It was difficult to take that one on the chin because it could have been averted quite easily.
"I could see that the lads had taken their foot off the pedal before they scored - and that can't happen again this season.
"It's one of those games I'll look back on at the end of the season and say we were our own worst enemy. The second half performance is something we'll have to stamp out if we're going to achieve anything."
Glenn lamented the second half performance that saw the team squander and good first period and a lead against Newcastle.