Sir Alex Ferguson left Old Trafford ahead of Manchester United's Premier League clash with Liverpool on Sunday after feeling unwell, sources have told ESPN.
The 84-year-old former United manager arrived at the stadium before the game, but left before kickoff after receiving medical attention.
Ferguson, who retired in 2013, regularly watches the club's matches from the director's box.
United's legendary manager took over the team after leaving Aberdeen in 1986, leading the club to 13 Premier League titles and two Champions League triumphs. In total, he lifted 38 trophies in his 1,500-game United tenure as the team established itself as English football's dominant force.
Ferguson suffered a brain hemorrhage in May 2018, and after surgery, he received a standing ovation when he returned to Old Trafford the following September.
United have been contacted for comment.
On Sunday, United held off a Liverpool fightback with Kobbie Mainoo's 77th-minute goal sealing a 3-2 win that means the club will play in the Champions League next season.
Information from PA and the Associated Press contributed to this story.