Byun has been rested from training this whole week since the results of an electrocardiograph test revealed he had left ventricular hypertrophy (enlarged left ventricle) on Tuesday.

The condition is similar to the ones which claimed the life of Cameroon football international Marc Vivien Foe during a Confederations Cup match in 2003, and Scottish representative Phil O'Donnell, who collapsed and died while playing for Motherwell in 2007.

Just how serious Byun's condition is yet unknown but the Jets are taking no chances with the 32-year-old Korean.

"He has got an issue with his heart. He has been to cardiologist this morning as he had a bit of an abnormality come back and we are going through the process of making sure that everything is okay," Jets coach Gary van Egmond revealed.

"It is to do with his left ventricle – it is enlarged and the medical staff are taking a lot of precautions to make sure his health is paramount.

"He found out about this week and we will know more exactly when the tests are back. Once we've just got the results and once we had those results we acted upon it as quickly as possible."

Depending upon the results of further tests Byun might be cleared to play in this weekend derby against the Mariners.

However, if the condition is serious, the defender may need to be fitted with a pacemaker similar to the one that saved the life of 20-year-old footballer Anthony van Loo, who collapsed with a heart condition but was jolted back to consciousness when the pacemaker reset the correct rhythm of his heart.

"If everything is alright we will keep Byun in the team but we just don't know," said van Egmond, who was looking at a number of alternatives to filling the left back role should Byun be rested.

"Ruben (Zadkovich) has played as a fullback with Olyroos, Ben Kantarovski has played in that position, Jacob Pepper is one we have been looking at as a fullback and there is Taylor Regan and well, it is just a case of finding the best fit.

"His health comes first of course. We have to get to the bottom of it to make sure there is no health risk."