Davies, aged 21, and McCusker, 23, have both been named in this summer's Wales squad which will tour North America. Jones will be joined by Scarlets teammates Mark Jones and Josh Turnbull.
Wing Jones misses the Arms Park Magners League clash with a knee injury. Back from injury in his place is wing Darren Daniel. Scarlets Lions Matthew Rees and Stephen Jones start on the bench and Simon Easterby returns as a replacement following surgery. Easterby's captaincy role is taken for the night by Lyons.
Scarlets head coach Nigel Davies said: "I'm very pleased for the Scarlets who have been named in the new Wales squad, particularly for the first-timers Josh, Rob and Jon. This season we've talked a lot about the development of our younger players and them having to stand up for us and it's been great that they've responded so well.
"Between them all they've played a lot of games and as a result they've made it into the national squad."
For McCusker there was more reason to celebrate this week as he signed a two-year extension to his Scarlets contract. Jonathan Davies signed an extension in March.
Davies said: "Rob has been around our set-up for a few years and this season has shown new maturity, has developed a genuine edge to his game, has contributed well to the group and now understands the game really well. He's somebody with a very bright future."
An Arms Park win is important to the Scarlets as they fight to secure automatic Heineken Cup qualification for next season. Their only game after this is on Saturday at home to Ulster.
Davies said: "We go to Cardiff with a strategy and with a strong, competitive 22. We should show a real hunger and desire because we've got everything to play for. "
Having secured a win against the Dragons last Wednesday the Blues will be looking to notch up another victory over Welsh rivals when they take on the Scarlets.
Speaking ahead of the match coach David Young said, "The Scarlets are fighting for their lives with the Dragons to avoid the play off match in Italy and I imagine they'll come here hoping that we'll roll over because the job is done for us. We never want to lose any games at home and especially to our Welsh rivals and I expect it to be another hotly contested Welsh derby."
Teams: Blues 15 Ben Blair 14 Richard Mustoe 13 Jamie Robinson 12 Gareth Thomas 11 Chris Czekaj 10 Ceri Sweeney 9 Richie Rees 8 Maama Molitika 7 Robin Sowden-Taylor (capt) 6 Ben White 5 Deiniol Jones 4 Bradley Davies 3 Faao Filise
2 Rhys Thomas 1 John Yapp
Replacements: 16 Gary Powell 17 Ross Johnston 18 James Down 19 Scott Morgan 20 Darren Allinson 21 Dai Flanagan 22 Nicky Robinson
Scarlets: 15 Daniel Evans, 14 Darren Daniel, 13 Rob Higgitt, 12 Jon Davies, 11 Lee Williams, 10 Rhys Priestland, 9 Gavin Cattle, 1 Phil John, 2 Ken Owens, 3 Deacon Manu, 4 Lou Reed, 5 Dominic Day, 6 Dafydd Jones, 7 Rob McCusker, 8 David Lyons (c). Reps - 16 Iestyn Thomas, 17 Matthew Rees, 18 Aaron Shingler, 19 Simon Easterby, 20 Martin Roberts, 21 Stephen Jones, 22 Ceiron Thomas.
