Thomas made the honest assessment as the Ospreys prepare for Saturday's intriguing clash with Leinster in Dublin - a match which sees the 2008 champions host the 2007 tournament winners at the RDS.
The Ospreys were the only Welsh region to reach the knockout stages of the Heineken Cup this season and finished second in the final Magners League standings, yet Thomas insists that anything less than a win later this week would leave the region feeling shortchanged.
"Our goals at the start of the season were to win in Europe and win the Magners League," said Thomas.
"By our own standards, we will be disappointed this season if we don't pick up some silverware.
"Our season will be judged, from outside, as a failure if we don't win…as a squad, we will feel we have underachieved."
As far as challenges go, taking on the 2009 European Cup winners in their own backyard has to be close to as tough as it gets.
Leinster boast a 100-per cent-record at the RDS in this year's Magners League and haven't been beaten at the ground in Celtic competition since September 2008.
But, while Thomas recognises the enormity of the task ahead, the Wales flanker cum second row insists confidence is high in the Ospreys camp.
"Dublin is a tough place to go and we are playing the former European champions" added Thomas.
"It will be hard but we are confident of what we can do on the road - and we won't be daunted by the challenge.
"We know it's not going to be a pushover but we are confident in our ability."
Just being in the inaugural Grand Final is an achievement in itself when you consider how the Ospreys began their Magners League campaign.
Two home defeats in the opening three weeks of the season was hardly the start one of the tournament favourites had been looking for.
Having been as low as seventh in the standings at one point in the season and in danger of missing out on the Play-Off places altogether at the end of March, Thomas admits it has taken a while for the players to get used to the influence of new director of coaching Scott Johnson.
"I have definitely seen a difference in us this year in the way we play the game.
"Scott has come in and everybody knows what he can do as a coach, though we knew it would take time for everybody to get used to his methods.
"He has a unique coaching style and it takes people a while to understand his concepts.
"Early on it was difficult and we lost a couple of games but when we drew with Leicester at Welford Road we came together as a team.
"We have progressed as a side but we still aren't where we want to be."
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