Thomas, who had filled in during Nick Robinson's absence, will miss at least three weeks after damaging the AC-joint in his right shoulder during last weekend's defeat to Leinster.
It is yet another blow for the Blues who are already down to a skeleton team amid a catalogue of injuries and will leave coach Dai Young with third-choice fly-half Nick MacLeod for Monday's local derby against Newport Gwent Dragons at Rodney Parade.
Thomas said: "It couldn't have come a worst time, I'm gutted. I'd had a run of games in the starting team and there are some crucial fixtures coming up. This is going to be a very frustrating Christmas."
The Blues will also be without centre Jon Bryant, who made his debut for the club on Saturday, under the conditions of his move from the Dragons last week. However Tristan Davies made his return from a hand injury against Leinster and looks set to start.
The WRU has teamed up with Crawshay's to tour Georgia in June. The side, which will be called the Crawshays WRU President's XV, will play Emerging Georgia twice, in a tour which supports the Welsh Government's efforts to grow relations between the two countries.
Wales Sevens are looking to re-group after a disappointing tournament last time out in South Africa. The next double leg in New Zealand and Las Vegas offers Gareth Williams a chance to redeem themselves.
Wales Women's head coach Rhys Edwards was at Llandovery to see the conclusion of an entertaining Women's Regional championship where any one of three teams were in contention for title glory on the last day of action.
Christmas has come early at Glynneath rugby club, one of a number around Wales who have received an array of merchandise and equipment this December courtesy of the Welsh Rugby Union's £750,000 participation points scheme.
The WRU have been delivering on the scheme - which gives individual clubs access to up to £5,000 worth of supplies and equipment each for the 2014/2015 season - over the last two weeks and today (Friday) were received by a special guest in Glynneath, club president and pitch marker Max Boyce.
As the countdown continues for RWC2015, Tournament Chief Executive Debbie Jevans visited the Millennium Stadium to discuss preparations for the eight matches and reveal the positive news that 100,000 tickets for games at the home of Welsh rugby have sold in the last seven days.