United lost the game 2-1 and they were chasing an equaliser when Spurs keeper Hugo Lloris came racing out to meet Young, who was charging into the area.
Young got to the ball first, sending an effort towards goal, but was then sent tumbling to the turf by the on-rushing Frenchman.
The winger was knocked off his feet after clear contact as he attempted to avoid Lloris, yet Webb remained unimpressed, rejecting United's penalty claims - much to Moyes' disgust.
"It was scandalous, a scandal," said the United boss. "If you follow through on a player anywhere else on the pitch with your foot high, it would be a sending-off and a red card. You couldn't do that anywhere else.
"The goalkeeper comes out, Ashley Young gets the ball before him and he follows through.
"It's an incredible decision which didn't go our way, in fact probably the one of the worst I think I saw."
Moyes' mood was not improved by Adnan Januzaj's booking, seemingly for diving, for the third time this season. In mitigation, the challenge from Danny Rose which preceded the fall was a firm one.
"Have you seen that decision? That is a terrible decision," said Moyes. "Adnan has got bumped. The boy has tried to edge him off the ball.
"All I can tell you is that (Lloris' challenge on Young) is a stonewall penalty kick. If that isn't given a penalty kick, I don't think we will get any more in the league this season.
"It is reckless, it is late, it is in the penalty box. I don't know what else you can say.
"You'd hope the people who put the referees out there would have a look at it and see if they think the referees are doing as well as they can be."
The defeat was United's fourth at home in the Premier League this season, which leaves them 11 points behind leaders Arsenal.
"I am not frustrated," said Moyes. "I thought we played really well. Today was a defeat of course, but we deserved to win, or at worst draw."
The return of Wayne Rooney to the starting line-up was one piece of good news for Moyes, but he admits the striker may have to be used sparingly in the coming weeks because of his groin issues.
"If I could (rest him), I would," said Moyes. "But the games are coming thick and fast.
"He has missed a couple with his groin and may well miss some more in the games coming up."
Moyes said it was too soon to know if Robin van Persie, who has missed the last six games with a thigh injury, will be fit to return for Sunday's FA Cup tie against Swansea
"I'm not sure yet," noted Moyes.
Moyes insists though that the latest setback will not affect his January transfer window plans.
"The result today would never have any effect on what we do, whether in January or the summer," he said.
"It wasn't our day but it won't change the way we go about our business."