A statement from the Old Trafford side read: “Sir Alex Ferguson has undergone emergency surgery today for a brain haemorrhage.
“The procedure has gone very well but he needs a period of intensive care to optimise his recovery.
“His family request privacy in this matter.”
United players past and present sent messages of goodwill to Ferguson, with his former goalkeeper and captain Peter Schmeichel tweeting: “Please be strong. Win this one.”
Defender Luke Shaw added: “Sending my love and best wishes to Sir Alex” while Jesse Lingard wrote: “Thoughts and prayers, Sir Alex.”
Ashley Young wrote: “Gutted to hear the news tonight about Sir Alex. Don’t really know what else to say other than thoughts and prayers with you and your family, Boss.”
Former United striker Mark Hughes, speaking after Southampton’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.”
Nicola Sturgeon, the First Minister of Scotland, said: “My thoughts are with Alex Ferguson and his family - wishing him a full and speedy recovery.”
Aberdeen issued a statement reading: “The thoughts and prayers of everyone connected with Aberdeen Football Club are with our former manager, Sir Alex Ferguson and his family following tonight’s news.”
Neil Simpson, who played under Ferguson at Aberdeen, added: “Get well soon Sir Alex. Wishing you a speedy recovery.”
A St Mirren statement said: “Everyone at St Mirren sends our best wishes to Sir Alex Ferguson and pray for a speedy recovery.”
Dunfermline Athletic, who Ferguson played for between 1964 and 1967, released a statement reading: “Everyone at DAFC is thinking of Sir Alex Ferguson at this difficult time.”
The Scottish FA tweeted: “Everyone at the Scottish FA wishes Sir Alex a strong recovery” while a message on the St Johnstone Twitter account read: “Best wishes to former Saint Sir Alex Ferguson from everyone at St Johnstone Football Club. “We are all hopeful of a full and speedy recovery for you.”
Celtic tweeted: “Everyone at Celtic wishes Sir Alex Ferguson a full and swift recovery following emergency surgery today.”
A tweet from Chelsea’s official account read: “This evening, everybody at Chelsea FC sends our best wishes to Sir Alex Ferguson for a full and swift recovery following emergency surgery.”
His son Darren Ferguson, manager of Doncaster Rovers, missed his side’s final game of the season against Wigan Athletic due to “family reasons”.
A club statement read: “Rovers manager Darren Ferguson will not be at Saturday’s game due to family reasons.
“[Assistant manager] Gavin Strachan will oversee the final game of the Sky Bet League One season in the manager’s absence.
“Darren asks for privacy, and will provide an update through the club during the week.”
Ferguson retired in 2013, after nearly 40 years in football management.
He began his coaching career with East Stirlingshire in June 1974, aged just 32. He was offered the role on a part-time basis, receiving £40 a week in salary.
Just four months later he was invited to take charge at St Mirren. The Buddies were, at the time, below The Shire in the league but were a bigger club, and Ferguson took the job after seeking advice from Jock Stein.
He led St Mirren from the lower echelons of the old Second Division to the First Division title in 1977.
However, he was sacked by the Love Street club, supposedly for various breaches of contract.
Former St Mirren chairman Willie Todd confirmed in an interview in May 2008 that Ferguson had been relieved of his duties because he had breached his contract by agreeing to take the reins at Aberdeen.
In 1977, Ferguson turned down the chance to manage the Dons, with Billy McNeill appointed instead. When he returned to Celtic a year later, Ferguson did take the Pittodrie job.
Under his stewardship, Aberdeen won the UEFA Cup Winners’ Cup, the UEFA Super Cup, four Scottish Cups, three league titles, one League Cup and one Drybrough Cup.
He remains the last manager to win the Scottish league title with a non-Old Firm side.
After a brief spell in charge of the Scottish national team, following Stein’s untimely death, he joined Manchester United in late 1986, after being linked with the managerial roles at Tottenham Hotspur, Liverpool and Wolves.
Despite a difficult start to his tenure, he eventually transformed United into a footballing powerhouse, and departed in 2013 having won 38 trophies with the Red Devils, including 13 Premier League titles, ten Community Shields, five FA Cups, four League Cups, two Champions Leagues, one UEFA Cup Winners’ Cup and one UEFA Super Cup.