Down to 10-men thanks to Isaac Hayden's 43rd minute red-card, Toon capitulated at the King Power and Lascelles wasn't happy with the display.
"It was terrible from the boys and we have no one but ourselves to blame," he told Sky Sports. "Leaking four goals even with 10 men is not good enough and it's just not acceptable.
"I don't think I've been here before where we've conceded five goals. We should've made it hard for them. Teams have been doing it throughout time, so it's not good from us.
"It's never happened like that. The boys do care, and having a man down is a disadvantage but we could've rolled our sleeves up and made sure they didn't score again but we didn't get into shape.
"It was nowhere near good enough. You should have a bit of pride in yourself and win your duels and battles. The simple things that you're taught, we didn't do.
"I don't want to say that we gave up, but we need to sit down and look back at that as it is nowhere near acceptable. The gaffer won't let us have our heads down too much, and we will need to let today go."