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Sainsbury (hamstring) and Luongo (ankle) were ruled out with 18 year-old De Silva and 24 year-old Croatia-based defender Mrcela drafted into the group that will assemble in Dubai on 8 June before the 17 June (AEST) qualifier in Bishkek.
West Australian born Mrcela left Australia as an 11 year-old and after spells at numerous Croatian clubs has been a consistent performer for top flight side Lokomotiva Zagreb - playing 29 times this season for the fourth placed outfit.
Teen sensation De Silva played 18 games for Glory in his final A-League season, scoring three goals, and has long been on the radar at an Australia youth international.
"It is a shame to lose Massimo and Trent but it gives an opportunity for two young players who we have been watching for a while in Tomislav and Daniel," Postecoglou said.
"Daniel is a prodigious talent who has been progressing well and after watching him play for the Young Socceroos against the likes of Brazil and Mexico he has earned a chance with the Socceroos.
"Tomislav has put together two strong seasons in Croatia and he is a big strong central defender we think can add something to the squad."
After it's opener against Kyrgyzstan, Postecoglou's side will also face Jordan, Tajikistan and Bangladesh in Group B of the Asian Football Confederation's second round of qualifying for Russia 2018.
