Midfielder Jack Colback says Newcastle lacked ruthlessness during their 0-0 draw with already-relegated Aston Villa on Saturday.
The Magpies will join Villa in the Championship next term if Sunderland win one of their two remaining games.
"We never really looked under threat defensively, but we never really had that killer instinct going forward," Colback told BBC Newcastle.
"I can't really answer the question as to why - we knew how big the game was."
The 26-year old midfielder added: "It's difficult to put your finger on it. We knew how important it was. I don't know whether a bit of fatigue kicked in or what, but we weren't quite as energetic as we have been."
The draw extends Newcastle's unbeaten run to five matches, their best sequence of results since November 2014 - but their survival is now not in their control.
Protests from frustrated Villa supporters throwing beach balls onto the pitch briefly halted the match, something Colback thinks affected his side's chances.
"Obviously the beach balls and inflatables getting put on the pitch made it difficult," he said.
"We didn't really get momentum in the game. It was stop-start."
Newcastle have one match remaining against Tottenham on Tuesday, 15 May, but could already be relegated by the time they play if the Black Cats can earn three points at home to Everton on Wednesday.
"Everyone's gutted. It's obviously out of our hands now but we just have to pray Everton do us a favour," the former Sunderland player continued.
"They're a good side, so there's no reason why they can't go to Sunderland and get a result.
"Then we'll back our home form to get a win, and then you're looking at Sunderland's result again."