Without a host of first team regulars Spurs controlled possession but fell behind to Wayne Rooney’s goal late in the first half before a second half Ashley Young double secured all three points.
Jermain Defoe emerged as a late substitute to claim a consolation goal, but the defeat means Harry Redknapp’s side have stumbled to back to back defeats in the Premier League for the first time since August.
King believes Tottenham deserved to take something from the game, and bemoaned the defensive lapses which helped contribute to all three of United’s goals.
"It’s tough to take because I thought for the majority of the game we gave a good account of ourselves,” King told Tottenham’s official site.
“But if you fall asleep at vital times you will get punished.
“We conceded the first goal at a bad time. That was disappointing, a free header in the six-yard box shouldn’t be happening.
“We tried to regroup, tried to come out and perform again, tried to lift ourselves and we did that, but went to sleep on a throw-in.
“You won’t beat teams like United unless you are fully concentrated for the full 90 minutes, and that was a bit of a lesson.
“You can’t fault our work-rate but it’s those moments that make all the difference."
Tottenham now face an F.A Cup fifth round replay against Stevenage on Wednesday before difficult looking Premier League games against Everton, Stoke and Chelsea.
Spurs now sit just four points clear of Arsenal in third place having seen there ten point advantage disappear thanks to the form of the Gunners, while any faint hopes the north London club harboured of a title challenge appear to have gone up in smoke with only their second defeat of the season at White Hart Lane on Sunday.