"I'm not going to go too much into that, but the players had a bit of a chat in the changing room after with no staff around," Shaw told Sky Sports.
"Because it's down to us players on the pitch, and it was nowhere near good enough. We had to speak out in front of each other."
Newcastle leapfrogged Manchester United into third in the Premier League standings with the win and exacted a measure of revenge over their visitors after a 2-0 loss to them in the League Cup final in February.
"We were disappointed with the final," said Callum Wilson, who along with Joe Willock scored Newcastle's goals on Sunday at St James' Park. "It left a sour taste in our mouth. We had to put it right in front of our fans."
They made it right in fine fashion, outmatching their opponents in virtually every position on the pitch.
Manchester United's poor performance was tough to stomach, said a visibly angry Shaw, especially coming after the international break.
"Not what we wanted. Not what we were expecting. When you come here to a tough stadium and a very good side and you don't come with the right attitude and hunger it will be a tough day. That showed today. We deserved to lose," he said.
"The whole team had a bit of rest," Shaw added. "Our levels had dropped, that was clear to see. Coming in today we felt good and ready for the game. The whistle goes and it's the complete opposite to what we've been doing in training. Unacceptable."
The defeat means Manchester United are fourth in the standings, only a point ahead of Tottenham Hotspur in the race to seal a spot in next season's Champions League.
Newcastle simply wanted it more, Ten Hag said.
"I hate to say it but they were better today – especially their determination, passion and desire. They wanted to win more so they won," the Dutch coach said.
