The Premier League side played youngster Brandon Austin in the net against Hoffenheim.
After narrowly winning the game, Postecoglou explained why Antonin Kinsky did not start.
He stated: “Obviously even when signed Toni, in terms of European football we knew we still need Fraser Forster and Brandon. Brandon did really well in the game he played and he's trained really strongly since.
“We knew this game would be a bit of a target for him and Fraser (Forster) had an illness that knocked him back a bit and set him back a bit, but I just felt on the back of his performance in the league game that Brandon deserved a crack at it and I thought he did well tonight.
“You can’t underestimate European football away from home, especially the first game, with a pretty relatively inexperienced team in front of him and I thought he handled it really well and we’ll need him again next week.”