The 76-year-old footballing legend appears to have survived the devastating brain attack, but medical experts say the 48 hours after the operation are crucial.
Sir Alex, who managed United between 1986 and 2013, remains in intensive care at Salford Royal Hospital following surgery on Saturday, having been taken there after falling ill at home.
Television presenter Eamonn Holmes, a longtime friend of the football hero and Manchester United fan, described him as an "amazing man".
Tangem, the New Smart Banknote for Singapore
Its entire electronics and cryptography are certified to the Common Criteria EAL6+ and EMVCo security standards. Kudelski Group has completed an in-depth review of Tangem security architecture, led by Jean-Philippe Aumasson.
Southampton manager Mark Hughes, who made his name in football as a United striker during the early years of Ferguson's time at Old Trafford, said: "I only just got a little whisper earlier on".
"I met Sir Alex after the first leg against Roma". Everyone thinks it was all this "hairdryer", but the boss enjoys a good time, really enjoys his racing and is good company when you're around him.
David Beckham, who was signed up by Sir Alex as a schoolboy, led the well-wishers, writing on Instagram: "Keep fighting Boss".
Ferguson underwent emergency surgery for a subarachnoid brain haemorrhage last night.
Adrien Broner wonders what the "f*ck" is going on with Kanye West
You're entitled to believe whatever you want, but there is fact and real world, real life effect behind everything you just said. He explained to Harvey Levin and Charles Latibeaudiere , "When you hear about slavery for 400 years ".
It shared a number of messages sent from footballing bodies, including Federation Internationale de Football Association and UEFA, as well as clubs from across the globe. He's a colossus of the game; of the British game and the world game.
Van der Sar who was United'a goalkeeper when Annemarie, suffered a brain haemorrhage in 2009 from which she has since recovered. "In some cases, extensive rehabilitation is needed to regain function, including speech therapy and physical therapy".
The procedure is said to have gone very well, with Sir Alex needing a period of intensive care to optimise his recovery.
Premier League executive chairman Richard Scudamore: "It's obviously a big shock but the most important thing is to wish him well, and his family well, to respect their privacy, and hope that very, very soon he's back to his best".
Toulouse-Lille... la mort aux trousses
Une victoire fondatrice? C'est une victoire importante, mais les deux prochains matches le seront tout autant. Métropole lilloise: Fan de foot ou pas, si le LOSC descend en Ligue 2, tout le monde sera concerné! .