Aston Villa manager Gerard Houllier was recovering in hospital on Thursday after being taken ill the night before.
Villa confirmed Houllier will be kept in hospital for several days while he undergoes a series of tests, so assistant boss Gary McAllister takes charge for Saturday's home Premier League clash with Stoke.
The 63-year-old, who is believed to have checked himself into hospital, is in a comfortable condition and was well enough to speak to the club's chief executive Paul Faulkner.
Although Villa didn't reveal the nature of Houllier's problem, the Frenchman had open heart surgery in October 2001 while he was managing Liverpool, after suffering severe chest pains during a match against Leeds.
"The club can confirm that Gerard Houllier was admitted to hospital last night," a Villa 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 is currently undergoing tests. It is anticipated that he will remain in hospital for several days. The club's thoughts at this time are with Gerard and his family."
Faulkner added: "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."
When he took the position at Villa, Houllier revealed he had consulted his heart surgeon before accepting the job.
He said he had returned to speak to Dr Abbas Rashid, the man who led the team who performed an 11-hour operation on the Frenchman, and was given a clean bill of health.
He has had a rocky ride this season with some Villa supporters calling for him to be sacked, but results have improved in recent weeks and Villa have climbed into the top half of the Premier League.
Current Liverpool boss Kenny Daglish revealed that his club's former doctor was at the hospital with Houllier.
"Of course our thoughts are with him," Dalglish said. "We don't know too much about it and there is no point in us being speculative but your health is more important than anything, so we wish him well.
"Doc (Mark) Waller is there, who was the club doctor here when Gerard fell ill in 2001, so he's got a good guy beside him and I'm sure the people there will do everything they can to help."
Tony Pulis, manager of FA Cup finalists Stoke, also sent Houllier his best wishes ahead of Stoke's visit to Villa Park this weekend and admitted the pressure of leading a top Premier League club can get to anyone.
"Everyone in football will wish Gerard a swift recovery, we hope he is well. I don't know how bad it is but I am sure he will get well looked after," Pulis said.
"Pressure is part of the job, you get used to it. Irrespective of what medical care you get, nothing stops you from getting excited on the touchline or carrying the pressure on your shoulders if things aren't going too well."
"If it's just happened, Gerard will have the team prepared and Gary McAllister works closely with Gerard so there shouldn't be too much disruption for them."