The Portuguese star caused controversy after he tapped the ball into an empty net from 12 yards after Tottenham goalkeeper Heurelho Gomes had put the ball down to take a free kick.
Spurs manager Harry Redknapp and his players remonstrated with the officials, saying they thought the referee had blown for a free kick after a Nani handball.
However, referee Mark Clattenburg had signalled to play on, and after a discussion with his assistant, the goal was allowed to stand.
Nani told reporters: “That was the strangest goal I think I've ever scored, most people were angry but I was clever because I understood the game.
“The keeper put the ball on the floor and took two steps back. I looked at the referee and he just said, ‘Yeah, OK’, so I took it quick and scored. It was not my fault.”
United boss Sir Alex Ferguson says he hopes Monday's game is not tarnished by a similar incident.
He said: “For the sake of Harry Redknapp's health I hope we can avoid the kind of controversial goal that last season had him jumping up and down in rage and frustration.”