Aston Villa manager Gerard Houllier will be kept in hospital for several days after being taken ill on Wednesday.
The nature of the problem remains unclear but the club confirmed Houllier is "comfortable" and the club's chief executive, Paul Faulkner, said he sounded "very positive".
An official club statement on Thursday confirmed assistant Gary McAllister will take charge of the team for Saturday's game against Stoke.
"The club can confirm that Gerard Houllier was admitted to hospital last night," the statement read. "He is in a comfortable condition and he wishes to convey his appreciation to Aston Villa supporters for their concern and good wishes.
"The manager spoke to chief executive Paul Faulkner this morning and he is currently undergoing tests.
"It is anticipated that he will remain in hospital for several days. Assistant manager Gary McAllister has taken training this morning and he will be in charge of the team for the game against Stoke on Saturday at Villa Park.
"The club's thoughts at this time are with Gerard and his family."
Faulkner added: "On behalf of the football club, Mr Lerner and I wish to convey our best wishes to Gerard. I spoke with him this morning, as did Gary before he took training, and he sounded very positive.
"I've also received a lot of good wishes from fans and I would like to thank them for their kind thoughts in wishing Gerard a speedy recovery.
"I will see him this afternoon, along with Gary, and we will pass on the fans' messages. I know they will mean a lot to him. We would ask everybody to respect his privacy and that of his family at this time."
Houllier was taken to hospital in October 2001 during his time in charge of Liverpool and had to undergo emergency heart surgery. He was absent for five months before returning to the club.
When Houllier accepted the Villa job at the start of the season, he revealed he had sought the advice of heart surgeon Dr Abbas Rashid.
"When you go into a job you know there will be pressure, that it will be hard work and you won't sleep every night," he said at the time. "You need to make sure your body is ready. I have checked that. I am much healthier and fitter now than I was before."
Houllier has come under fire from the Villa fans this season, although the 2-1 win over West Ham at the weekend has lifted the club into the top half of the table.