"I have said many times before the boss wasn't just the greatest and best manager I ever played under, he was also a father figure to me from the moment I arrived at the club at the age of 11 until the day I left,"
Beckham added on Facebook.
Beckham was one of the original 'Fergie Fledglings', progressing from United's youth set-up with Gary Neville, Paul Scholes, Nicky Butt and Ryan Giggs, and won six Premier League titles, two FA Cups and the Champions League while at Old Trafford.
Despite success with his club, Beckham's international career was quite different at the start.
"After '98, without him I would have found it virtually impossible to cope with the attention I was getting," added Beckham. "I will always be grateful to him for his support and protection.
Tributes to Sir Alex Ferguson
"He understood how important it was to play for your country and he knew how much it meant to me."
Neville, who played his whole professional career under Ferguson, told Sky Sports News: "As a fan, I'm sad and gutted. As a player, I'm grateful and privileged to have worked with the greatest manager of our time.
"It's just a loss all around. I was shocked, but you have to respect his decision."
Neville, now a successful football pundit, added: "If you have 499 employees at Manchester United you had 499 'mini-mes' of him. He's the last of a kind.
"He really maintained traditional values and a family atmosphere. He immersed himself in Manchester United and has taken them to a point of dominance now.
"It's always been a day that Manchester United fans have dreaded but the club will move on without Sir Alex Ferguson."
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