The England international comes up against his former club for the first time since leaving Old Trafford seven years ago, but says the past will be put to one side for one night when the two sides take to the field at the San Siro in the last 16 of the Champions League.
"It is going to be an emotional game, but as a Manchester United fan, it's the first time I am going to want United to lose," he said.
"As an AC Milan player, I want to beat Manchester United and to say that as a Man U fan is quite difficult.
"As much as it is emotional for me personally, it is important that we win the game.
"I am a Milan player and I want us to play well in the game and progress to the next stage."
Beckham rejected the suggestion that he will be particularly motivated to get one over Sir Alex Ferguson on Wednesday night.
Beckham left Old Trafford under something of a cloud in 2003, but insists he only has good memories of his time with the Red Devils.
"Sir Alex will always be a father figure to me," he said. "He always has been and he always will be. No matter what has been said in the past, whether good or bad, I only remember the good times.
"He was the man who gave me my chance to play for club I had always dreamed of playing for.
"For me they were only good times. Sir Alex is respected throughout football all around the world.
"Clubs move forwards and it was my time to leave Manchester United in 2003, and I moved on.
"I definitely hold nothing against the manager, he's an incredible man."
Beckham's place in the Milan team on Tuesday night is not guaranteed and his presence at the pre-match press conference does not necessarily suggest that Milan coach Leonardo is ready to change the team he has used for the last two league games in which Beckham has sat on the bench.
"His appearance here today is important for us and I had asked him personally to come," explained Leonardo. "He is somebody who represents this club and he is here because of his experience and charisma.
"He's a very smart guy, a polite guy and an incredible player.
"If you look at his career you can see that he has done everything in football, not only on the field.
"He's a complete personality and he proves every day why he's so important for us."
Despite the praise, though, Leonardo may indeed have to overlook the 34-year-old as he seeks to find a way of beating United.
"Not conceding is always an objective, but we are also a team who try to score goals and to change our identity gets complicated," said the Brazilian coach. "This desire to score goals is inherent; it is our identity.
"To try and find the balance between not conceding and to hurt them I believe is ideal."
Alexandre Pato is set to return to the Milan attack while Klaas-Jan Huntelaar should replace the injured Marco Borriello with Ronaldinho completing the three-man attack.
Central defender Thiago Silva appears to have recovered from a groin strain he picked up early in last Friday's Serie A game against Udinese and he should partner Alessandro Nesta at the heart of the defence.
Beckham, should he start, could replace Gennaro Gattuso in the midfield alongside Massimo Ambrosini and Andrea Pirlo.