Ferguson did not find success in his early years at Old Trafford and some pundits believe that in today's cut-throat management world, he would not have been given the time to find his feet.
Ferguson famously won the Treble of Premier League, FA Cup and Champions League in 1999, the year in which he was knighted.
Messages of support and well wishes have come in from footballers all around the world and Guardiola took time to send his best wishes to his former rival and his family.
On Instagram, with a picture of him as a youngster alongside Ferguson, Beckham wrote: "Keep fighting Boss".More news: HSBC Q1 profit misses estimate, unveils $2 billion new share buyback
When Ferguson retired in 2013, he was the most successful manager in British football.
Club captain Michael Carrick said: "Absolutely devastated to hear about Sir Alex being unwell in hospital".
In a later tweet, the club also wished Ferguson a "comfortable, speedy recovery". All my thoughts and prayers are with him and his family.
Ferguson's family have requested privacy as he recovers in Salford Royal Hospital. All my thoughts and prayers are with him and his family.
Liverpool said it would offer its full support to Manchester United and Ferguson's family.More news: #BellewHaye2: Tony Bellew Knocks Out David Haye To Win Rematch
He added: "All my thoughts and prayers are with him and his family".
Ferguson "brought greatness back to United", said former Stretford End favourite Denis Law, a key figure in the celebrated United teams of the 1960s.
It is just per week since Ferguson had been about the Old Trafford pitch having to pay tribute to long-time managerial adversary Arsene Wenger. Headway, the brain injury association, explained a hemorrhage are caused by a ruptured aneurysm or perhaps a setback on the mind, which include a slight accident.
"The long-term implications are very hard to predict", Headway's Luke Griggs said.
There was a heartfelt message from former United goalkeeper Edwin van der Sar and his wife Annemarie, who herself recovered from a brain haemorrhage in 2009.
FC Barcelona too wishes a speedy recovery for Sir Alex.More news: Lava from volcano eruption in Hawaii destroys two homes
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