Nottingham Forest's Carling Cup tie at home to Leicester was abandoned at half-time after Foxes defender Clive Clarke collapsed in the dressing room.
Paramedics rushed down the tunnel to attend to the 27-year-old shortly after the half-time whistle.
"Clive was very seriously ill," said Leicester chief executive Tim Davies.
"Our thoughts and prayers are with Clive and his family." Clarke was in a stable condition in a cardiac unit at Nottingham's Queen's Medical Centre.
Davies added: "I would like to say a big thank you to Nottingham Forest for dealing with the matter so fast."
Forest were leading the Carling Cup tie 1-0 at half-time thanks to Junior Agogo's goal.
However, after a delay of 15 minutes, Forest boss Colin Calderwood and Leicester manager Martin Allen emerged from the tunnel to announce to the crowd that the game had been abandoned.
Calderwood said: "We are afraid that due to the serious illness of a Leicester player both clubs have agreed to postpone the match."
Forest chief executive Mark Arthur said later: "I think football is secondary to human issues.
"As soon as we found out the serious nature of Clive's situation, we went straight to the referee and said we would do whatever is right by Leicester.
"Obviously, they are going through great trauma at the moment and we very quickly agreed between the three parties that the game should be abandoned."
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