But Blues coach Dai Young insists talismanic number eight will win his race against time for the crunch Euro tie against Gloucester on October 19.
Young said: "Xavier's injury is a big blow. He is a vital player for us for us but his knee flared-up after the Connacht game.
"We now hope that Xavier will be back for our game with Gloucester at the Millennium Stadium the weekend after next."
The Blues will also be without Wales wing Tom James who has a groin problem but British Lion Gethin Jenkins and Tom Shanklin return.
Blues: Ben Blair; Leigh Halfpenny, Jamie Robinson, Tom Shanklin, Gareth Thomas; Nicky Robinson, Jason Spice; Gethin Jenkins, T Rhys Thomas, Gary Powell, Deiniol Jones, Paul Tito, Maama Molitika, Martyn Williams, Andy Powell.
Replacements: Taufa'ao Filise, Gareth Williams, Scott Morgan, Robin Sowden Taylor, Richie Rees, Ceri Sweeney, Jamie Roberts.
Referee: Jerome Garces (France)