British media reports that this is a preventive measure, not a medical emergency.
Ferguson, who spent 27 years at Manchester United and won 38 trophies, regularly watches the club's matches from the official box.
According to the BBC, the former coach was seen at the stadium earlier in the day, in good spirits, a few hours before the start of the match.
Club officials expressed optimism that Ferguson would soon be released from the hospital and return home.
He had a serious health problem in 2018 when he suffered a brain hemorrhage, and in 2021 he spoke in detail about his recovery.
Fergie was at Paris Saint-Germain's Champions League semi-final against Bayern Munich on Tuesday.
