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.
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.