But the Newport Gwent Dragons' emphatic victory over Cardiff Blues handed the Ospreys a life-line and it is one Thomas is determined to take full advantage of.
"The changing room after the Munster game was a hugely disappointed one - we felt it was the end of our season - so to have a second chance and be in a shout of qualifying is great," said Thomas.
To qualify for the Play-offs and have a chance of defending their Magners League crown, the Ospreys must beat Aironi and hope the Scarlets beat fourth-placed Cardiff Blues in Llanelli.
The Dragons could also do their Welsh rivals a favour by denying Ulster anything at Rodney Parade.
"We have to hope other results go out way," added Thomas, who started last season's Grand Final victory over Leinster at the RDS.
"I just hope the Scarlets do a job on the weekend and also the Dragons. The way they've been playing at home, there's no reason why they can't beat Ulster by more than seven.
"But it's tough to talk about it without knowing how results will go. We just have to focus on our job.
"When we found out the Blues had lost to the Dragons there was that flicker of hope and there's been that extra spring in our step this week.
"But we have to step up on the weekend and put in a good performance - if we don't qualify then at least we've done our part of the bargain."
And if things do go their way in Round 22, Thomas remains confident that, despite their recent mixed form, no team would want to face the Ospreys in the knockout stages.
"If we're present at the semi-finals all the boys will be really determined to go out and do a massive job. Who knows what can happen then," added the Wales international.
"You can ask any team, no one enjoys playing us. We're a physical side and Munster will not want to face us in the semi-finals."
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