This is the day that Manchester United fans have feared for so long that it feels like the end of the world. Sir Alex Ferguson is retiring as manager.

Most of his squad was not even born when Fergie arrived at Old Trafford from Aberdeen in November 1986 and set about “knocking Liverpool off their perch.”

To simply catalogue his achievements and a record breaking haul of 38 trophies in 26 years that have made him the most successful British manager of all-time is only part of this epic story.

The Governor from Govan is not just a man, not just a legend – but simply the best.

He has not merely won trophies. The Emperor has built a dynasty. He has restored pride and glory to the most famous football club in the world. He has created a never-say-die spirit that gives the Theatre of Dreams a mystical aura that is indefinable. During Fergie’s reign his successes on the pitch have been reflected in the transformation of Old Trafford into one of the world’s greatest stadiums – the Mecca for a global following who worship the Red Devils.  He has made generations of players past and present feel a unity and a belonging that is unique in the game. He is the godfather who oversees the family that is Manchester United.

We all knew that this day would come. But that does not diminish the feeling of shock that is reverberating around planet football as United’s vast army of fans in every corner of the world come to terms with the news.

Peter Schmeichel  – who won five domestic titles and the  Champions League, in United’s Treble winning year of 1999 – summed up the feelings of many when he tweeted: ‘Disappointed, shocked, sad. Didn’t think THAT day would be today.’

The voice of MUTV, my old pal Paddy Crerand was audibly shocked when he went on BBC 5 Live and declared: “I’m just stunned. God help the poor fellow that’s going to have to follow him because the standard he has set is incredible.”

United’s chief executive David Gill, who said working with the Scot had been the “greatest experience” of his career, paid tribute by adding: “What he has done for this club and for the game in general will never be forgotten.”

Fifa president Sepp Blatter tweeted: “His achievements in the game place him without doubt as one of the ‘greats’. It was an honour to present Sir Alex with award at 2011 Ballon D’Or. Will his longevity at the top ever be repeated?”

Paul McGrath, who played under Ferguson during his early days at Old Trafford, simply tweeted: ‘Sir alex ferguson . Respect .!’

Michael Owen – who recently admitted his career would have lasted so much longer if he had started out at United and not Liverpool – said: ‘What a privilege to have played under arguably the best manager the world has ever seen.’

Paul Ince explained why the timing has been such a surprise: “I was totally shocked. What he has done is unbelievable. You can’t explain winning all those Premier League titles not to mention the Champions League, FA Cups – he has done the lot and you will never see anyone of his kind again. Two weeks ago he was talking about staying on for another two years so it has come as a massive shock.”

The sense of shock was also reflected on by Dwight Yorke who said “Is the timing ever right for Sir Alex Ferguson to retire? I don’t think so. I think he has really taken the football world by surprise.”

The tributes from around the world will be endless and #ThankYouSirAlex will trend on twitter for days to come.

When the great man walks out for the final home match of the season against Swansea City on Sunday, for the last time  as Manchester United manager at Old Trafford, it will be one of the most emotional afternoons in the history of the club. It will be the day Fergie collects the Premier League title for an unbelievable 13th time, making it a record-breaking 20th League title in all for the Red Devils. It will be a day to reflect not just on the achievements of Sir Alex in winning trophies. But also to say a huge thank you to a man who has built an extraordinary Empire.