Sir Alex Ferguson, who led Manchester United to 38 trophies during 26 years in charge, has had emergency surgery on Saturday for a brain haemorrhage.
A United statement said the procedure "had gone very well but he needs a period of intensive care to optimise his recovery".
Ferguson, 76, retired as United manager in May 2013.
He was at Old Trafford last Sunday, when he presented Arsenal boss Arsene Wenger with a commemorative trophy.
The most successful manager in the history of the British game, Ferguson's trophy haul at Old Trafford included 13 Premier League titles.
Former Manchester United striker and Southampton manager Mark Hughes, who won two Premier League titles under Ferguson, said: "I heard some whispers and was hoping it wasn't true. I wish him all the best."