Cullen waits to hear how long O'Brien will be out with shoulder injury
The British & Irish Lions tour is beginning to look like a costly adventure for Leinster and even Ireland.
Centre Robbie Henshaw is already on the long road, that is four months, back to full fitness from a pectoral muscle completely detached from the bone.
The failure of Henshaw to beat the Champions Cup return-to-play deadline by mid-October would leave a hole in Leinster's midfield where Garry Ringrose would probably be paired with Noel Reid.
It is an area of meagre resources, in terms of numbers, and there could even be a temptation to use Joey Carbery there in what would be an intriguing combination with Jonathan Sexton.
Mind you, the movement of Carbery between out-half and full-back didn't do him any favours towards the business end of last season.
More immediately, Sean O'Brien was on the receiving end of a hammer blow from Jerome Kaino at a firs t-half ruck in the third test.
The flanker lasted until half-time where the loss of power to his shoulder caused his removal.
He will have his shoulder scanned to estimate the period of recovery for a man not immune to long-term injuries.
This comes as a sour note at the end of a wonderful tour for O'Brien, just pipped for the Lions Player of the Series by Jonathan Davies.
"We tested him at half time and he just didn't have any power," said coach Warren Gatland.
"He's had an operation on there before. We'll just see how he turns up tomorrow.
"We'll probably look to get him scanned and see. I think he's got a piece of his hip in there. They replaced some bone in the past."
This is where Leinster are far better equipped to handle bad news about O'Brien.
The Ireland tour to the United States and Japan last month revealed how Jack Conan is ready to throw down the gauntlet to Jamie Heaslip for province and country.
There has also been the senior contract upgrade awarded to Max Deegan, the 2016 U20 World Player of the Year in a position that boasts an embarrassment of riches.
The leadership of captain Rhys Ruddock was highly praised and the impact of Josh van der Flier and Dan Leavy was impressive.
Then there is the return from long-term injury of Jordi Murphy and the graduation of Peader Timmins to the senior squad.
In fact, none of the four Ireland starters in the British & Irish Lions third test in New Zealand made it to the finish line.
While O'Brien gave way to injury, Conor Murray was replaced by Rhys Webb, Tadhg Furlong by Kyle Sinckler and Sexton by Ben Te'o.
The form and fitness of Sexton will be greeted with delight by Leo Cullen and Stuart Lancaster.
The master of his craft is right back to the form that can lead The Blues confidently into Europe.
Tight-head Furlong's consistency convinced Gatland he is better than Sinckler and Dan Cole, at the moment.
This will further inflate the confidence of the 24-year-old as he moves into the exalted company of being a target for opposition front rows.