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 Wales Women's team headed to Dare Valley Country Park on a team bonding exercise as the entire squad got together for the very first time in preparation for this season's Six Nations championship which kicks off on Saturday with a clash against defending champions Ireland, in Donnybrook.
Wales Head Coach Warren Gatland discusses his team selection for the opening round of the RBS 6 Nations where he leads his side against Ireland, on Sunday. As WRU TV's Luke Brodley discovers, Gatland is fully aware of the challenges the defending champions pose
WRUTV caught up with Wales Women captain Rachel Taylor at the RBS 6 Nations launch and how a good crowd really helps boost the girls performance. Tickets for all three Women's home games at The Gnoll, Neath are available from www.wru.wales/tickets
The WRU Coach Core Apprenticeship scheme is developing young people who are already benefitting Welsh rugby while improving their own career prospects. WRU TV caught up with some of the individuals who are making a difference.
The Wales Sevens squad had some spare time to kill while waiting for a training session in Dubai, so decided to play: Let's head a football into a rubbish bin. It seemed such a simple idea at the time....