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's Women and Girls legacy events - #TRY OUR GAME have proved popular this week with current players and newcomers to the game learning new skills and picking up tips from national squad players.
We caught up with Shona and Ellie from the Wales women team ahead of the start of the Women's Rugby World Cup in Paris next month at a recent women's training session to encourage and promote more women and girls to try their hand at rugby. #tryourgame
The Women's Rugby World Cup warm-up clash between Wales and USA proved bittersweet for centre Elen Evans. Despite being one of the star performers on the day on her 50th Test appearance, USA edged home 10-7 at Cross Keys.
Wales completed their Junior World Championship campaign with a victory over Samoa. Team manager Mark Taylor and No 8 James Benjamin reflect on a tournament which saw Wales win three games and lose two.
Wales U20 players Angus O'Brien, Nicky Thomas and coach Richard Hodges are determined to finish their Junior World Championship with a flourish when they tackle the big hitting Samoans tomorrow in Auckland.