Sir Alex has often made huge sacrifices to achieve success. Today, he overtakes United's other great Scottish boss, Sir Matt Busby, for length of service as Reds manager.
“It’s an incredible achievement," says Phelan, who has worked with Sir Alex as a player and as a coach, so he is well placed to pay testimony to his talents.
"His secret is his enthusiasm, his attention to detail, his work ethic – that’s something he was born into, which comes from his background and his family.
"He is youthful, has a young mind, and he's always willing to learn.
"But he’s always lived knowing that to achieve anything you have to work at it and put the hours in. He’s certainly done that. You see signs of that every day.”
Message board: What do you think has been Sir Alex's greatest achievement?