The Manchester United boss will leave the job after 26 years.
“I spoke to Sir Alex recently and I had no inkling that he was considering retiring from his position at Manchester United,” said Big Sam. “So, like for everyone else, it has come as a bit of a shock.
“I think it’s clear he was going to announce this at the end of the season, but whispers have got out somewhere, so he probably had to make it official right now.
“For himself, his wife Kathy and the rest of his family, I hope they can enjoy his retirement, the time they will have together and see things they have always wanted to see.”
“Sir Alex has had an unbelievable career, not just at Manchester United, but when he first started at Aberdeen. For me, he is the greatest club manager that has ever been produced, certainly in this country, if not Europe.
“However many managers there were before him, the huge task he had was to achieve what Sir Matt Busby had done before him. For him to have done that and more, to have won the treble for Manchester United, and the joy and satisfaction of doing that gave was fantastic.
“To build as many teams as he has built over the seasons, on and off the field, was brilliant. He changed the backroom staff to support what he was trying to do and as a man, he is a fantastic human being.
“We will all sadly miss him, many others will too. Manchester United now have a massive task to replace him.”