Ryan has missed five matches for Valencia and was ruled out of the Socceroos' 2018 FIFA World Cup qualifiers against Bangladesh and Tajikistan last month after injuring his knee in a club match against Deportivo La Coruna on 30 August.
Ryan was forced to undergo surgery the next day to repair a tear in his right meniscus.
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Ryan could miss four weeks
The 23-year-old was on the way to a complete recovery and had already started light training when his knee flared up again last week.
Club doctors said the troublesome right knee had become infected and Ryan was taken to hospital for treatment, according to local newspaper and radio reports.
He is now back home but he has been ruled out of Valencia's league match against Malaga at the Mestalla on 17 October, the resumption of league football after the FIFA break.
At this stage he has a battle on his hands to be selected for Australia's November World Cup qualifying matches against Kyrgyzstan in Canberra and Bangladesh in Dhaka, although the latter fixture is in doubt because of security concerns in the capital city.
Ryan, who was a member of Australia's 2015 AFC Asian Cup-winning team in January, helped Valencia reach the group phase of the UEFA Champions League after joining the club from Club Brugge in the European off-season.
