Here, Angus Wright takes a look at some of the social media reaction.
David Beckham, perhaps the most high-profile former player of Ferguson’s United reign, offered his support, posting a picture on Instagram of him signing a contract as a teenager alongside Ferguson.
Accompanying the picture was the message: “Keep fighting Boss.. Sending prayers and love to Cathy and the whole family x @manchesterunited”
United club captain Michael Carrick said he was “absolutely devastated” by the news, while Mike Phelan, who played under Ferguson for United and later served as his assistant, wished his former boss a swift recovery.
Former United forward Wayne Rooney, who now plays for Everton, gave his support to the Ferguson family, tweeting: “Get well soon Boss. Thoughts with all the family at this sad time. #AlexFerguson”
Goalkeeper Peter Schmeichel, who was part of United’s treble-winning team in 1999, posted: “Please Be strong Win this one”, and Ashley Young, whom Ferguson brought to Old Trafford in 2011, spoke of his shock.
“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,” he said on Twitter.
Sports figures from outside of football also passed on their best wishes.
Golfer Justin Rose said the thoughts of all of the European Ryder Cup team were with Ferguson, a keen follower of the sport.
Fellow golfer Ian Poulter shared a photograph on his Twitter page of him sitting side-by-side with Sir Alex.
He wrote: “Thoughts and prayers go out to Sir Alex Ferguson and family.. wishing him a speedy recovery.”
Using the hashtag “legend”, Olympic champion Sir Mo Farah said: “Get well soon Sir Alex Ferguson my support are with family and friends..!!”
Meanwhile, the Red Devils’ City rivals united following the news of Ferguson’s operation.
“Everyone at Manchester City wishes Sir Alex Ferguson a full and speedy recovery after his surgery #footballfamily” said the club, while Arsenal offered words of camaraderie, having seen Ferguson help give long-serving manager Arsene Wenger a fitting send-off at Old Trafford.
“We’re all thinking of you, Sir Alex,” the Gunners said on Twitter, alongside a picture of the pair from last weekend.
Aberdeen, where Ferguson secured the Scottish league title three times and also the 1983 UEFA Cup Winners’ Cup, offered their support, saying: “Everyone connected with Aberdeen Football Club are with our former manager, Sir Alex Ferguson and his family following tonight’s news.”
Former England striker Gary Lineker, the host of BBC’s Match of the Day, added: “Very sorry to hear the news that Sir Alex Ferguson is seriously ill in hospital. Wish him all the very best.”
Ferguson also received good wishes from Scottish First Minister Nicola Sturgeon, who wished him a full and speedy recovery.
The official account of the Premier League, where Ferguson guided United to such success with 13 titles, offered its thoughts, while football’s world governing body FIFA also sent words of support.