Lock Andrew Coombs says being forced to withdraw from the Wales side to face Tonga was the "toughest day" of his career.
The Newport Gwent Dragons captain was forced to withdraw hours ahead of the game with a back injury.
Coombs, 29, says missing the Tonga win cost him the chance to stake a claim for a place against Australia.
"It was really disappointing and probably the toughest day I have had in professional rugby," said Coombs.
Coach Warren Gatland only had British and Irish Lions locks Ian Evans and Alun Wyn Jones fit and available for Wales' 30-26 defeat to the Wallabies.
The six-times Wales lock or back row player would have been in contention for a starting place against Australia, along with Evans and Jones.
Wales lost Bradley Davies to a shoulder injury against South Africa, while Luke Charteris returned to his club Perpignan in France because the Wallabies clash was outside the International Rugby Board's recognised Test window.
Coombs revealed he trapped a nerve in his back during a Wales scrummaging session ahead of the Tonga game.
"I had a trapped nerve in the middle of my back, so every time I moved it would catch and there was no way I could play," said Coombs.
"It's part of the game, getting injuries, but you knew if you had played well you had a chance to play in the Australia game.
"It was a tough campaign for me, training for five weeks and getting no reward."
Coombs has returned to lead the Dragons in their Amlin Challenge Cup clash with French side Begles-Bordeaux on Friday.
He is determined to make up for lost time with his region and prove himself with the Dragons in Europe.
"Now I am back here [the Dragons] and there is a big focus now on playing well," said Coombs.
"We have a new boy Cory Hill and he knows all the calls and I have seen his some of his clips and he is really good.
"I have to play well this week to keep myself in here [the Dragons].
"I am really keen to get out and play because for five weeks I have been training hard...and not playing."