Gabor Gyepes suffered heart palpitations during Pompey’s midweek defeat to Leyton Orient, it has emerged.
The Hungarian was out-of-sorts during the Fratton Park match and was substituted in the 35th minute, Ricardo Rocha replacing him.
Afterwards, Guy Whittingham was unsure what was wrong with the central defender, although he ruled out an injury.
However, it has since been discovered Gyepes was suffering from palpitations and a high heart rate.
Pompey are now awaiting results from a check-up as they seek to assess the full extent of the problem.
Speaking at the end of Tuesday night’s 3-2 defeat, though, Whittingham admitted Gyepes’ situation was not clear.
He said: ‘There’s no injury, Gabor just didn’t feel right.
‘We asked several times if he was all right and we didn’t really get a clear answer.
‘Should we have substituted him earlier? I suppose in hindsight you can kick yourself and think “if we did it five minutes before would they have scored that goal?”.
‘You are trying to liaise with the player, you are not really finding out what is up with him. Then, as soon as that goal went in, you realise a substitution has got to be done straight away.
‘Gabor was struggling for whatever reason. It’s not an injury, we know that. He just didn’t look well and there was no warning beforehand. He didn’t tell us before the game.’
Gyepes has made 16 appearances for the Blues since arriving in September.
During that time he has been one of the more consistent performers.