LSir 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.
Ferguson's family have requested privacy as he recovers in hospital.
The most successful manager in the history of the British game, Ferguson's trophy haul at Old Trafford included 13 Premier League titles.
United's club captain Michael Carrick said he was "devastated" to learn his former manager had undergone emergency surgery.
"All my thoughts and prayers are with him and his family. Be strong boss," he wrote on Twitter.
Ferguson began his playing career with Scottish club Queen's Park as a 16-year-old striker whilst working as an apprentice tool-worker at Clyde Shipyards.
His most notable spell as a player came in a two-year stint at Rangers from 1967. He retired as a player in 1974 when he was on Ayr United's books.
He began his managerial career as a 32-year-old at East Stirlingshire before going to St Mirren, where he won his first trophy by taking the Scottish first division title in 1977.
Ferguson moved on to Aberdeen and turned them into a major force in a Scottish top division where Rangers and Celtic had dominated.
He led them to three Scottish titles, four Scottish FA Cups, one League Cup and the European Cup Winners' Cup in 1983 by beating Real Madrid 2-1 in the final.
Ferguson managed Scotland in the 1986 World Cup in Mexico following the death of Jock Stein, although he was unable to take his country past the group stages.
He became Manchester United manager later that year.
More to follow.