His Senate office would only confirm that Demers, 72, was at home Sunday, but didn't provide any details about the reason he went to hospital.
Demers had suffered a stroke in April, but he was at the Canadiens' home opener Tuesday night, smiling in a wheelchair while handing a torch to Captain Max Pacioretty to close out a pre-game ceremony.
Demers led the Canadiens to their last Stanley Cup in 1993. He also coached the Quebec Nordiques, the Detroit Red Wings and the St. Louis Blues.
Demers was appointed to the Senate in 2009 by Prime Minister Stephen Harper, but later left the Conservative caucus in December to sit as an Independent.
At the time, he said he was uncomfortable with some of the fallout from the Senate expenses scandal and didn't like to have to vote the Conservative party line all the time.