Cavani had the ball in the net in the first half at the Tottenham Hotspur Stadium, believing he had fired the Red Devils into a 1-0 lead.
But, after consultation with VAR, referee Chris Kavanagh ruled out the strike for a foul by Scott McTominay on Heung-min Son in the build-up.
A flailing arm from McTominay caught Son in the face, and the Spurs forward stayed down as play continued and Cavani stuck the ball in the net.
Replays appeared to show the contact from McTominay was minimal and accidental, but after viewing the incident again on the pitchside monitor, Kavanagh ruled out the goal.
Sky Sports presenter David Jones read out a statement from PGMOL at half-time, in which they said: “It wasn’t part of McTominay’s natural running movement and was careless.”
To add insult to injury for United, Son scored at the other end just a few minutes later to put Tottenham in front.
Both former United skipper Roy Keane and ex-Manchester City defender Micah Richards labelled the decision “embarrassing”.
Keane said: “I’m amazed really. If this is a foul we should all go home. It’s really bizarre. A player like Son to roll around like that it’s embarrassing.
“That can’t be a foul. How a player can stay down? And it’s not just Son, to be fair Rashford done it about 10 minutes earlier. Both sets of players rolling around, screaming.
“Referee’s under huge pressure. They’re doubting every decision they give now but clearly I think he’s got this wrong.”
Richards added: “It’s embarrassing. This is not football anymore. I know we have a laugh and a joke and all that but this is spoiling our game.
“I can’t recognise the game anymore. If this is a foul, Roy’s right, we might as well as go home now. It’s an absolutely ridiculous decision. It’s spoiling football.”