Wednesday night's (Thursday AEDT) clash in London is the Socceroos' last match before the coach settles on his extended 30-strong World Cup squad in May.
The 21-year-old was Postecoglou's chosen keeper when the team defeated Costa Rica in November but faces stiff opposition from Mitch Langerak, Eugene Galekovic and Brad Jones.
Ryan spoke to reporters after Australia's first training session was cancelled due to heavy and admitted he had been dreaming of selection in Brazil.
"It's the pinnacle for any footballer," Ryan said.
"It's hard to not think ahead because it's only a few months away now.
"If I can maintain the level that I'm at the moment, I like my chances of being on the plane to Brazil," he said.