Leinster were brought down by one of their own in Jono Gibbes, the La Rochelle head coach who helped the Blues to win the Champions Cup three times.
The Kiwi now has the chance to win the Top 14 and European double before leaving for Clermont and Ronan O'Gara steps into the hot-seat. Toulouse stands in his team's way on both fronts.
La Rochelle are a team made up of a hotch-potch of nationalities, in stark contrast to Leinster - who started yesterday with 15 Ireland internationals.
But ex-Leinster forwards coach Gibbes believes nationality has nothing to do with success.
"I’ve a lot of experience with different squads, been lucky enough to work in a few different environments and in all honesty, it doesn’t come down to nationality or where you're born," he said.
"It comes down to who you are and what kind of people you’ve got.
"I remember Shane Horgan sticking up for Isa (Nacewa) when everyone was giving out about foreigners at Leinster.
"He said, 'we don’t have non-qualified players, we just have Leinster guys here'.
"I think that Shaggy quote, albeit a long time ago, has proved itself to be true.
"Yeah, we’ve got different nationalities but so what? Leinster have got 21 (homegrown players) out of 23.
"I guarantee you that those 21 guys are good lads, they're good people, they just happen to be born in Leinster, which works for them.
"The best environments I’ve been in, it’s not about that, it’s about whether you want to be here, whether you want to contribute and make the place better and leave it in a better place after you've gone.
"I was lucky enough to work in Clermont and certainly that was the experience I had there. Leinster, I’ve already spoken about that experience and how important it was.
"I just think it came back to the quality, quality people again. That’s what we started here.
"We moved a few people out that didn’t have the ambition, didn’t want to contribute and we brought people in and it started to turn around in the right direction."
It was Aussie second row Will Skelton who was Leinster's on-field Euro bogeyman as the Blues saw their Champions Cup dreams blown away by the Bay of Biscay.
Skelton, a thorn in the province's side in Saracens' final victory in 2019, was at it again at the Stade Marcel-Deflandre.
His presence helped Victor Vito to score the killer first try for La Rochelle in the 66th minute - and he helped himself to the second, leaving Ross Byrne's 77th minute effort a mere consolation.
Cullen first encountered the Aussie on a watching brief to the Waratahs and the Leinster supremo said: "He's caused us a bit of trouble when he was with Saracens as well - in the final in Newcastle, in particular.
"He's a huge man and can cause a lot of damage.
"We contained those guys well in the first half but it was just off the back of some penalties and field position when they got a bit of a roll on and a couple of guys slipped off a few tackles, particularly on Will.
"The game at this level can be quite simple, once you get on the front foot with momentum - and the half-backs controlled the game well for La Rochelle."
A quarter way through this first meeting of the clubs, Leinster looked good for Twickenham in three Saturdays time and that clash for a fifth star with fellow four time winners Toulouse.
By the end, however, the result was not in doubt.
Tadhg Furlong's seventh minute converted try was a decent return from a flying start - poles apart from the hammering Leinster took early on at Exeter in the quarter-finals.
“I thought in the first 20 minutes that Leo (Cullen) and Stuart (Lancaster) had done a real number on us with video during the week," grinned Gibbes.
"We just couldn’t buy a trick."
The absence of skipper and driving force Johnny Sexton wasn't a problem at that stage and Ross Byrne looked assured in his first start at this level.
But as the first half progressed, La Rochelle grew in confidence. The tide was turning.
Out-half Ihaia West was a calming presence, Brice Dulin's monster book kicked his side into good positions and O'Gara's defensive system locked Leinster in.
By half-time, the Blues were lucky to be 13-12 ahead, with the hosts missing the chance to score a try before the break.
Leinster still had their noses in front with 25 minutes to go but were clinging on as the hosts' pack got on top- and Skelton in particular.
“He's 150 kilos and moves and understands rugby and is a hell of a guy," said Gibbes.
"He carries those players around him and brings a lot of confidence to the pack.
"He didn’t have a specific tactical input with the Saracens background, but his actions during the week and all season have brought a big confidence to the group.
"He was a big contributor."
