He joined the Swans in the summer of 2009, debuting against Scunthorpe in the Carling Cup in August.
Idrizaj also had loan spells with Luton Town and Crystal Palace while at Liverpool, having been named Austria's Young Player of the Year before signing on at Anfield.
He went on to make three Championship appearances, two as a replacement, without scoring for the Swans and had a year left on his contract.
Idrizaj made no senior Liverpool appearances, but scored a 26-minute hat-trick at Wrexham for the Reds.
If confirmed as a heart-attack, Idrijaz's death follows several other similar deaths that have hit the sport in recent years.
In June 2003 Cameroon's Marc-Vivien Foe died aged 28 after collapsing on the field while playing against Colombia in Lyon, France having spent the previous season on loan from Olympique Lyon at Manchester City.
In August 2007 Spanish international Antonio Puerta, 22, suffered a heart attack playing for Sevilla against Getafe and later died in hospital.
Two years later Espanyol skipper Daneil Jarque died aged 26 after suffering cardiac arrest following a training session.
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