Ryan, who joined the Spanish team last month, went under the knife on Monday to correct a problem with the meniscus in his right knee and the procedure was "satisfactory", Valencia said on their website (www.valenciacf.com).
The Singapore-owned club, who drew 1-1 at home to Deportivo La Coruna in La Liga on Sunday, did not say how long the 23-year-old would be sidelined.
"It's a blow for Mat to miss these games but he picked up a knock in Spain overnight and with the travel distance and the lateness of his club game we have decided to bring in Eugene Galekovic," said Australia coach Ange Postecoglou.
"Eugene is a more than capable replacement and he will join us in Perth. We will move ahead without Mat."
Adelaide United keeper Galekovic, 34, has won eight caps for his country but none since 2013. Bournemouth backup Adam Federici and Leyton Orient's uncapped Alex Cisak are Postecoglou's other options in the squad.
Asian champions Australia host Bangladesh in Perth on Thursday before travelling to Dushabe for the Sept. 8 Group B fixture against Tajikistan.
Australia opened the joint 2018 World Cup and 2019 Asian Cup qualifying campaign with a narrow 2-1 win in Kyrgyzstan in June.
Jordan, the other team in the group, are expected to be Australia's closest challengers for top spot, which guarantees passage to the next stage of Asian qualifying for Russia 2018.
(Additional reporting by Iain Rogers, writing by Patrick Johnston in Singapore, editing by Sudipto Ganguly)
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