Welsh preparations were dealt a huge blow when the pair were ruled out of the tournament, which starts next Friday, after suffering injuries in the warm-up win over Italy at the weekend.
“What I think the injuries will do is send Wales in under the radar,” Thomas told WalesOnline.
“It was always going to be a difficult group and Wales were always going to be under pressure but I think it’s a good thing if we’re written off now.
“We have been losing to England and Australia for too long now — it’s been happening on a regular basis. To go into a game with a different mindset and game plan might not be the worst thing.
“Wales always perform best as an underdog anyway and being written off by others could end up being a good thing.”
Wales, semi-finalists in 2011, must do battle with hosts England and Australia in a group containing three of the world’s top five ranked teams — with only the top two progressing to the knockout stage.
Thomas said the absence of Halfpenny, arguably the most reliable goalkicker in international rugby, may result in a different approach from coach Warren Gatland which could see Wales “less reliant on the boot”.
“I think the injuries could mean Wales end up playing a lot more rugby,” Thomas said.
“Leigh Halfpenny will be a big, big miss. Wales rely on him to keep them in games. Not only do other sides fear Leigh but he’s a massive confidence booster for Wales too.
“To not have him is massive but it might mean we are not as reliant on the boot as in the past. We do have players who can play rugby.”
Thomas believes the versatile Liam Williams, who has fitness concerns of his own having broken a foot in June, can step up and shine in Halfpenny’s absence.
“When you lose two key players it brings about an opportunity for others which is massive. Liam has been so unlucky in the last few years,” he said.
“He’s had to play second fiddle and be moved to the wing but he’ll be coming in now as first choice.
“He’ll be desperate to show what he can do. He has been an outstanding player for Wales and this time around it looks like the jersey will be his.”
(Writing by Justin Palmer; editing by Amlan Chakraborty)