Clive Clarke's agent has 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.
"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 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."
Wishing this ex West Ham player all the best ..
Yes, best wishes to him for a speedy recovery. He was incredibly lucky.
I guess this means that he won't be able to carry on playing.