Sir Alex Ferguson has undergone emergency surgery today for a brain haemorrhage, his former club Manchester United have announced.
The procedure has apparently gone 'very well' but he needs a period of intensive care to optimise his recovery.
The 76-year-old's family have requested privacy in the matter, United added.
United midfielder Michael Carrick said on Twitter: "Absolutely devastated to hear about Sir Alex being unwell in hospital. All my thoughts and prayers are with him and his family. Be strong Boss."
Ferguson's son Darren, manager of League One side Doncaster, did not attend his side's final home game of the season against Wigan due to his father's illness.
Former United striker Mark Hughes, speaking after his Southampton's side's draw with Everton, said: "I only just got a little whisper earlier on. I was hoping it wasn't true. I wish him all the best."
In 2016, Fergie praised Fleetwood Town's new £8 million training ground when he visited to officially open it.
“I always think ambition comes from people who lead clubs,” said Ferguson, who won 38 trophies in 26 years in charge at Old Trafford.
Messages of goodwill for Ferguson poured in from United players past and present.
Jesse Lingard said on Twitter: "Thoughts and prayers sir alex."
Luke Shaw tweeted: "Sending my love and best wishes to sir Alex."
Peter Schmeichel tweeted: "Please Be strong Win this one."
Nicola Sturgeon, the First Minister of Scotland, said on Twitter: "My thoughts are with Alex Ferguson and his family - wishing him a full and speedy recovery."
Ruth Davidson, the Scottish Conservative leader, tweeted: "So many people will be wishing Alex Ferguson well and sending their thoughts to his family tonight."
Everton manager Sam Allardyce said: "It is tragic. I am going to find out how he is as soon as I possibly can. Obviously I knew it was extremely serious when I knew his son Darren didn't attend his own game at Doncaster today.
"I hope he is in good hands and I hope the operation is a major success because as a personal friend...I hope he has a full recovery."