Former Manchester United manager Sir Alex Ferguson had emergency surgery on Saturday after suffering a brain haemorrhage.
A statement by United said the procedure “had gone very well” but Ferguson “needs a period of intensive care to optimise his recovery”.
Fergie, 76, retired as United manager in May 2013 after winning 38 trophies during 26 years in charge.
His family have requested privacy as he recovers in Salford Royal Hospital.
“We will keep Sir Alex and his loved ones in our thoughts during this time, and we are united in our wish to see him make a comfortable, speedy recovery,” United later said in a tweet.
The Scotsman is widely regarded as the most successful manager in the history of the British game, having won 13 Premier League titles, two Champions League crowns, five FA Cups and four League Cups.
He famously won the Treble of Premier League, FA Cup and Champions League in 1999, after which he was knighted by Queen Elizabeth II.
Ferguson has been married to wife Cathy since 1966.
The tributes have been pouring in from across the football world, with United captain Michael Carrick saying he was “devastated” to learn his former manager had needed emergency surgery.
“All my thoughts and prayers are with him and his family. Be strong boss,” he wrote on Twitter.
Five-time ballon d’Or winner Cristiano Ronaldo also took to Twitter to express his support for his former manager, saying: “My thoughts and prayers are with you, my dear friend. Be strong, Boss!”
Also, former England captain and United winger, David Beckham, wrote: “Keep fighting boss. Sending prayers and love to Cathy and the whole family.”