The veteran no.2 came in after a long-term injury to first choice keeper Guglielmo Vicario.
Forster made some good saves in a 2-2 draw in the Europa League group stages against AS Roma on Thursday.
Postecoglou stated post-game: “I don’t think he’s lacking in confidence.
“He is an experienced guy and has probably thought to himself that, that is why he is there, if needed to come in and be that reliable goalkeeper for us. It’s helped he’s already played this year, it wasn’t his first game.
“He’s played his part tonight and it’s just disappointing we weren’t able to capitalise on the football we played to get the three points. I don’t think he’s lacking in confidence, but always good for him to least pull off a couple of big saves for us tonight and get him back into he group.”
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