Devin Toner stock running low at worst possible time
Towering Leinster lock in danger of losing starting place despite hugely successful partnership with O'Connell
"Sometimes it takes a bit longer for guys to mature into their bodies." Leo Cullen's summation of Devin Toner's progress over the last few years couldn't be more apt.
Like a good wine, Toner has gotten better with age, but having finally cemented his place in Ireland's starting XV over the last couple of seasons, he is now under severe pressure to keep hold of it.
Joe Schmidt is likely to introduce several fresh faces for the meeting with Romania on Sunday before the pool games become significantly more difficult.
It was Declan Kidney who handed Toner his international debut as a 24-year old in 2010 but the giant second-row was unable to establish himself as a regular.
With the likes of Paul O'Connell, Donncha O'Callaghan and Donnacha Ryan ahead of him in the pecking order, he was forced to bide his time, but once it came he grabbed his opportunity with both hands.
The arrival of Schmidt to these shores, first as coach of Leinster and now Ireland, transformed Toner's career.
The Meath native has always pointed to Cullen and O'Connell as being major influences, but the progress that Toner has made under Schmidt's tutelage has been remarkable.
The main criticism that was aimed at him in his early days was that he lacked the aggression to match his size, but that has improved over time.
It's been a case of better late then never but in the last two years for Ireland, Toner has earned his place in Schmidt's side.
A key component of Ireland's back-to-back Six Nations title has been the set-piece, and at the centre of that has been Toner's formidable partnership with O'Connell.
"I don't think I would be the player I am today without having played with him," Toner said of O'Connell's influence on him earlier this year.
"I've learned so much off him. How he goes about his business and how he calls lineouts is eye-opening."
Being left out of last weekend's match-day 23 for the opening pool win over Canada could perhaps be read as a sign from Schmidt, but second-guessing the Kiwi often leaves you red-faced.
Nevertheless, the facts were that the in-form Iain Henderson was given the nod from the start while the fit-again and versatile Donnacha Ryan was named on the bench, and Toner was forced to watch from the stands.
It's not a vantage point that he has been familiar with in recent seasons but it is one that is threatening to become more regular.
Toner is expected to be given a chance against Romania to stake his claim for the crunch games against Italy and France, and should he do so, it will mark another milestone in his career.
The 29-year old is still waiting to make his World Cup debut but once it arrives, it will be fully deserved.
Despite Schmidt repeatedly insisting that Ireland are not looking beyond the next game, the coach will have pored over his starting XV for the games against Italy and France a million times in his head.
Six months ago, Toner would have been a certainty to continue in the second-row alongside O'Connell, but Henderson's recent dominant displays for Ulster and Ireland mean that it is becoming increasingly difficult to leave him out of the side.
In the latter part of his tenure in charge of Leinster (2000-03), Matt Williams was aware of Toner as a young player coming up through the ranks and in his eyes, it's all about getting the correct balance in the side.
"Dev is going to be looking over his shoulder going 'Well if I don't get going here, I might be out of this team and on the bench'. So Dev has got to perform," the TV3 analyst says.
"Dev is great in the air. He gives you another dimension at lineout time but he's not a dynamic ball carrier. If you've got Sean (O'Brien) back in the team - they didn't have him for a long time - all of a sudden there's your best ball carrier in the group.
"You've got to make sure that you stress to your players to just be yourself. Don't try and be someone you're not. It's about the balance that they give.
"You've got Paulie and Dev; Paulie is just great, a workhorse, great at everything. Dev won so much crucial lineout ball and improved the attack in the last couple of years.
"Will Henderson be able to do that? Will he give that lineout option? Will we recover what we lose if Dev drops out? That's what you've got to balance as a coach."
For all of Henderson's powerful displays, it can be easy to forget what Toner brings to the side.
At almost 7ft tall, his height is an obvious advantage and over the last couple of years, Ireland's lineout has benefited massively.
Henderson's partnership with O'Connell, although still in its infancy, has shown enough to suggest that it could compete with most second-rows but it remains largely unproven against world-class opposition.
In Toner's case, he is part of a winning formula, which poses the question: should Schmidt tinker with a partnership that has been a major part of both Six Nations successes?
In Keith Wood's mind, the answer is quite simple:
"I'd put my best players on the pitch. I think Henderson has matured as a player over the last 18 months," says the former Ireland hooker.
"His ability to shift his feet just at the point of contact is just fantastic, especially for a guy that size."
After his torrid spell with injuries, Ryan has also quietly emerged from the shadows and his ability to play in both the second-row and back-row makes him an attractive proposition for Schmidt.
Another part of Schmidt's selection headache is that the back-up second-row chosen on the bench must be able to make an impact.
There is no question that both Henderson and Ryan are more explosive players to come into a game, which may work in Toner's favour.
It's a decision which will cause Schmidt sleepless nights, but during a World Cup, it's exactly the kind of issue that a coach wants.
Toner was recently asked what he feels he brings to the side.
"The main thing would be set-piece, I think, trying to secure our ball, and steal it back a few times. But I think it's just kind of doing the simple stuff well," he said.
Come Sunday afternoon, Toner will be firmly in the spotlight but whether doing the simple things will be enough to retain his place in the starting XV remains to be seen.