Lancaster believes O'Brien will be a leader for Exiles and thrive in English game
Less than a year after being forced to concede the departure of one world-class back-row Lion and multiple medal-winning stand-out for club and country, Leinster have conceded defeat in the quest to retain another of equally stellar vintage.
And yet the reaction, in public naturally one of disappointment, but in private one rather more phlegmatic, if not ruthlessly pragmatic, reveals much about a province who are eager to plot the future, rather than pity the past.
Winning a Champions Cup and PRO 14 title double without either player soothes any sense of grave professional wounding, if not removing personal sympathy.
February 2018 witnessed the retirement of Jamie Heaslip which, although not a huge shock given his injury difficulties, still jarred with many supporters; 12 months on, the province have conceded defeat in their attempt to retain Seán O'Brien's services.
With his central contract due to be terminated, Leinster were unable and unwilling to match the reported €500,000 three-year deal offered by London Irish.
They coped without Heaslip and they will cope without O'Brien, too. Both men featured so prominently for a decade and more with their home province; Leinster will rely on others to prosper in their absence for the next ten years.
"When you lose a player of Seán's quality everyone's disappointed," admits senior coach Stuart Lancaster. "In an ideal world, you keep all your best players until the very end. But you have that cycle, don't you? It's the evolution of any playing squad.
"Jamie was a good example. Jamie goes and Jack Conan steps up. You have Caelan Doris behind him.
"In the same vein, you have people like Dan Leavy and Josh van der Flier. Beneath them you've got Max Deegan or Scott Penny. That's the nature of the production line here in Leinster and we're very lucky to have that.
"But I don't think anyone's underestimating the impact Seán has had. I've coached many great players, and he's certainly held in the highest regard in my mind, because of the way he conducts himself on and off the field, his leadership.
"Even though injuries have been frustrating for him, he's always in every meeting, always contributing, always adding value, showing leadership.
"It's obviously a disappointing one for Leinster but I think Seánie knows it, we all know it here, that life goes on and there are really good players in the back-row in Leinster who will step into the void."
Head coach Leo Cullen, who won the vast majority of his medals alongside O'Brien, had the privilege of both playing and coaching the Tullow man.
"As a team-mate, Seán was the sort of competitor you always wanted beside you," he said. "He is the kind of player who gives you the confidence that anything is possible - nothing fazes him and he always led from the front.
"For now, we hope to finish this season strongly and, after his disappointment of missing out on a number of big games last season through injury, we hope Seán can play a major part in Leinster's push for silverware in two competitions this season."
For his part, Lancaster would have liked to have coached the injury-prone O'Brien more.
"I've been two-and-a-half years now," adds Lancaster, "but Seánie hasn't been available through injury for various reasons.
"I'd love to have coached him more, certainly day in, day out, week in, week out, because his influence off the field has been huge since I've been here yet he's only played in probably half the games."
O'Brien's latest comeback has seen him appear in mercifully rude health; he was arguably the best performer in Saturday's success in Murrayfield and, with appropriate husbandry, there could still be plenty of mileage left in the Tullow Tank.
"After speaking with Declan (Kidney), it was obvious the ambition London Irish have as a club," he said. "I'm keen to roll up the sleeves and use my experience to help re-establish London Irish as a top club in England.
"It's a club that's been around for a long time and there's an obvious connection to Ireland with some greats of Irish rugby having previously worn the jersey."
Lancaster feels neither O'Brien's rugged demeanour nor injury profile will hinder the 31-year-old's new career.
"He's probably got the advantage that Declan Kidney probably knows him better than anyone, really, so I'd be surprised if they try and play him in every Premiership game or in every European game.
"I think they'll use him wisely but I think he'll have the same impact at London Irish as he has at Leinster, his presence will be felt both on and off the field."
"If they do manage him wisely, there's no reason from what I've see over the last few weeks that he can't continue to play for the next few years."
Until then, O'Brien is hungry to make up for missing out on Leinster's double drive last term. When he does leave, the world will still turn.