"That's why we give goals away. We did not do our jobs in positioning."
United had travelled to Tottenham four days after defeating Brighton & Hove Albion in the FA Cup semi-final at Wembley. They also played at Sevilla in the Europa League last week.
"I know they are not robots and it is a tough schedule. But when you are playing for Man United and playing away, you have to give (everything) on every occasion," Ten Hag added.
Tottenham interim manager Ryan Mason praised his team's character after they bounced back from a 6-1 thrashing at Newcastle.
"In football, you win, lose, draw, but the most important thing is that we are united and act as a team. I have a team that is willing to fight," said Mason after his first game in charge in his second spell as interim boss.
Son said their comeback was driven by anger after an "unacceptable" performance last time out.
"Today we wanted to bring a good energy," he said.