Carbery all set to ride to the rescue
Coach Cullen puts faith in 20-year-old out-half
Just when Leinster looked primed for a shot at reclaiming their status as a force in The Champions Cup, Jonathan Sexton's hamstring tightened up.
The Ireland out-half has dropped out and 20-year-old Joey Carbery will step into the spotlight in making his European debut.
The Athy man has just three starts from seven Leinster caps tucked into his belt.
It is what the coaches have seen on the training paddock as much as on the field of play which leads them to believe in a truly gifted individual.
It would appear the stars have aligned for Carbery.
All he has to do is make them sparkle with his twinkling feet and natural instinct.
What Leinster have lost from Sexton is clear. What they can gain from Carbery will not be known until this afternoon.
Rob Kearney expects to see what the rookie has already shown.
"I think from Joey we have seen his ability to attack the line, his pace, his footwork," said the Ireland full-back.
"Obviously, Johnny is a very compete player, so he doesn't have too many deficiencies.
"But, I think there are a few things that Joey can really bring.
"There's been a couple of times this season where we have seen that he does have that tactical awareness, and if he needs to manage a game, he can do that.
"For Joey, it's just that decision making and trying to manage the game and when to take on a man and when to potentially have some tactical awareness too."
Luke McGrath has held off the claims of Jamison Gibson-Park to partner up with Carbery.
It is also an important personal test for Tadhg Furlong in his journey to Ireland's number one tight-head prop.
Jack McGrath is favoured over Cian Healy for this one and Ian Nagle has earned the right to pack down with Devin Toner in the second row.
Josh van der Flier has edged out Dan Leavy, covering all three positions in the back row from the bench, and Jordi Murphy to complete a fiercely physical trio with Rhys Ruddock and Jamie Heaslip.
While Leinster have been drawn in one of the toughest Pools, they appear to have the best chance of all the Irish provinces of winning first time out.
Castres have been known to be cast aside in Europe, especially on their travels.
Still, coach Leo Cullen is wary of "some of the noises coming out of their camp.
"They feel they want to perform in Europe and they want to give it a good crack this year. We need to be on guard," he said.
"I saw their game against Clermont a couple of weeks ago and they were in that game for a long period.
"They're dangerous. They go hard at the breakdown. They've got pace.
"They've got guys who have good evasive skills as well that are able to punish you off the turnover ball. It's being aware of the threat."
However, Leinster come into this on the back of fine form in the PRO12 League which has been rewarded with five wins from six and joint-top slot beside Ulster. This is a case of Leinster taking care of their own business.
"We need to focus on what we can control as well ourselves," heeded Cullen.
"It's making sure we're as best prepared so we understand what we're facing.
"We know that if we get our parts right, we'll be hard to beat here in the RDS."
Leinster: R Kearney; Z Kirchner, G Ringrose, R Henshaw, I Nacewa (capt); J Carbery, L McGrath; J McGrath, S Cronin, T Furlong, D Toner, I Nagle, R Ruddock, J van der Flier, J Heaslip.
Castres: G Palis; R Grosso, T Combezou, R Ebersohn, D Smith; B Urdapilleta, A Dupont; A Tichit, J Jenneker, D Kotze, V Moreaux, R Capo-Ortega (capt), M Babillot, S Mafi, A Tulou.
leinster v Castres, Today (KO 3.15), live BT Sport 3