But they will travel to Ireland without their starting No2 after Thomas suffered an ankle injury in last week's 23-16 success against their Welsh rivals at Rodney Parade.
Thomas trapped his leg in a ruck and fell awkwardly just eight minutes in to the first derby of the new season.
The initial prognosis suggests the 30-year-old will be out of action for three months, although surgery isn't required.
That length of absence would see him miss the league fixtures against Zebre, Ospreys, Leinster, Glasgow and Connacht, plus both PRO12 games with Munster.
He would also sit out October's Amlin Challenge Cup ties with Mogliano and Bath and the December double header with Bordeaux Begles, as well as the LV= Cup games against the Scarlets and London Wasps.
His first involvement back with the matchday squad is likely to be one of the festive derbies against his old side Cardiff Blues in Newport on Boxing Day or in the capital on New Year's Day.
"It's a shame because Rhys was playing well and he was enjoying his rugby here," said Dragons boss Lyn Jones.
"He's one of those experienced characters who adds a lot on the pitch and on the training field as well.
"It is a blow but it's not the end of the world and he'll be surprised by how quickly the three months goes."
Thomas, who won 27 caps for Wales between 2005 and 2008, had been in impressive form since joining from Wasps in the summer, starting all three league games to date and skippering the side at Murrayfield in Round 2.
His misfortune is expected to lead to an extended chance for Hugh Gustafason, with Sam Parry set for his first involvement from the bench in Munster and Wales U20 star Elliot Dee waiting in the wings for his opportunity to step up.