Clive Clarke's agent revealed 'paramedics had to use a defibrillator' after the Leicester defender collapsed at half-time in the Carling Cup tie at Nottingham Forest.
The second-round clash was abandoned at the break after the 27-year-old had lost consciousness in the dressing room.
Clarke, who joined the Foxes on loan from Sunderland earlier this month, received treatment from paramedics at the City Ground but regained consciousness and was taken to hospital in Nottingham.
Agent Gary Mellor told BBC Radio Stoke: 'Clive is sitting up and talking and he is going to have more tests at the hospital on Wednesday afternoon.
'They won't give us any idea as to when he will be released. They don't want to tell us any more until they know exactly what happened.
'It appears Clive's heart stopped twice and mouth-to-mouth didn't work so the paramedics had to use a defibrillator.
'His family are very upset, especially after what happened with the Sevilla player Antonio Puerta dying recently. We just hope he's going to be OK.'
Leicester chairman Milan Mandaric insists that Clarke is making progress is his recovery from a double heart failure after the defender 'sat up and spoke' this afternoon.
Mandaric told Sky Sports News: 'Unfortunately I have not had chance to see him but it is encouraging news that he is stabilising and he has sat up and spoke and is quite comfortable and resting.
'It is important he just gets better. It is a sad thing when you see a young man collapsing and getting down on the floor.
'It wasn't something that anyone can accept easily but I think the most important thing now is that he gets out of hospital and gets on with his football.'
Leicester manager Martin Allen paid tribute to the club's medical staff.
He told the club's official website: 'I must say [head physio] Dave Rennie and [sports scientist] Damien Doyle were absolutely outstanding in coping and dealing with what was a traumatic and difficult situation.
'We are lucky both members of staff were on hand at such a difficult and testing time.'
Foxes chief executive Tim Davies said his 'thoughts and prayers' were with Clarke and his family.
Davies, who described Clarke as 'very seriously ill', said no one at the club was aware of the player suffering from any medical condition.
He said: 'When Clive went into the dressing room he collapsed and was very seriously ill. This is the first time we are aware of anything happening to Clive. He was taken away in an ambulance to hospital. He did offer a few words in the ambulance.
'Our thoughts and prayers are with Clive and his family, we hope he makes a speedy recovery.'
Sunderland chairman Niall Quinn has sent the club's best wishes to Clarke and has wished the Dubliner a speedy recovery.
He told the Black Cats' official website: 'Our thoughts and prayers are still very much with Clive and his family and we wish him a speedy recovery.
'We are in constant contact with the Leicester, who reacted magnificently in the circumstances. Sunderland AFC are very grateful to the medical departments at both Leicester and Nottingham Forest for all they have done in a very difficult situation.'
Sunderland manager Roy Keane added his thoughts to those of his chairman.
He said: 'He's one of our players and fingers crossed, please God, he will be okay. We wish him well.'
The abandoned Carling Cup tie has been rescheduled for September 18 and supporters who attended last night's match will be granted re-entry on production of their original ticket.
Forest chief executive Mark Arthur told the club's website: 'We would like to thank all those people who have contacted both ourselves and Leicester with messages of support for Clive.
'I would also like to apologise once again for any inconvenience last night`s unfortunate events may have caused but Clive`s well-being, as well as that of his team-mates had to come first.'