"There's a different feeling around the camp now because the competition has gone," said Thomas.
"We've all been there in any walk of life, in any situation when there is anybody vying for the same spot as you, but he's your mate, you're still competition.
"The trouble is, whatever was happening at all the training sessions, whatever we were supposed to do always ended up getting a little bit more tasty than it should have and with a little bit more enthusiasm than it should have because of the competition.
"The World Cup is the biggest thing in anybody's playing career so when you're competing for it, you're competing for it. You're not going to stand back and think 'oh, he did that well, I'll let him get on with it'. You are going to try to do better.
"The competition was there and now it's gone. Okay, the competition now is in some ways more heated because it's about starting XV spots, but the competition for getting on the plane has gone.
"You've taken the competition away and replaced it with another thing that isn't as fierce."
Thomas leads a powerful side into action against the Pumas and, while the Welsh public are demanding a dramatic improvement on the 62-5 drubbing from England at Twickenham on August 4, the 33-year-old insists all the pre-tournament games should be viewed in the context of the World Cup.
He said: "You've got to put the performance against England in context. There are two more games to play before the World Cup - against Argentina, then France. For instance, if we lose these two games and win the World Cup, who'd remember us losing to England, Argentina and France?
"There have been a lot of knee-jerk reactions. We didn't want to lose against England and not by that margin. The thing is that the reaction was based on the fact it was Wales v England and it just seemed to me that nothing else happened that Saturday, nothing else in the world."