Newcastle captain Bruno Guimaraes branded his side's defeat to Sunderland as "unacceptable" and "embarrassing".
Newcastle's 1-0 loss extended their winless Premier League run against their closest rivals to 10 matches in one of their poorest displays of the season.
The visitors managed just two shots on target and an xG of 0.25.
"We didn't perform the way we can perform," Bruno told Your Site. "It's frustrating, it makes me angry. Our performance was not there today - no shooting the ball, no crossing the ball, no passing.
"It's so frustrating for me to come here today but the fans are right to be annoyed because it's a tough one to accept.
"Everyone came here and knew exactly what it meant for the fans. The whole messaging in changing room was 'give it to the fans' and we didn't. It's so embarrassing.
"I'm very angry for the performance. We missed the consistency, which is not there, the mentality's not there."
Newcastle boss Eddie Howe added: "It's going to sting for a long time," Howe said. "Our performance was off of what we needed it to be. Not from a defensive perspective, we defended pretty well, but the one big moment went against us.
"Criticism comes with every defeat but with this one it'll be louder and stronger because of the importance of the game. That's why it hurts so much, I'm so disappointed for our supporters and sorry for the performance in terms of the lack of goalmouth action and clinical attacking which we've prided ourselves on in recent seasons.
"You never want one game to define a season but if anything you want to define it in a positive way and show your class. It's been a season of stop-starts for us, it feels. We're trying to get the momentum we want and we haven't grabbed it fully.
Sunderland man of the match Dan Ballard said the "hurt" of Newcastle taking a picture in front of the away end following victory at the Stadium of Light in their previous meeting in January 2024 prompted a revenge mission for the hosts this time around.
The Black Cats squad lined up as one following their win over their fiercest rivals, a tongue-in-cheek mirror of what Eddie Howe's side did after their own FA Cup win on Wearside almost two years ago.
Speaking to Your Site after the game, Ballard said: "That hurt last time. A lot of the players wanted to do it - but it is what it is. The most important thing was three points and winning today."
On those painful memories of their previous encounter, where Ballard's own goal was the first in Newcastle's 3-0 win that day, he said: "Words can't describe it. Thinking back to the last game, it was so painful.
"For the players, for the fans, it felt like we let them down. We were so determined this time. Especially the boys who were there and experienced that day.
"It prepared us more for today, we came into the game with a lot more quality in the team and some strong leaders.
"It was't the prettiest game but we stuck in there, we fought hard and thoroughly deserved the win."
