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The medics say the damage is similar to the trauma of a relatively heavy car crash, which sounds a bit extreme until the size and power of the athletes is factored in.
Nonu, at 1.82m and 108kg, is a serious weapon. Roberts, at 1.93m and 107kg, is a door of a man. Both are mobile, agile and elusive, but their clash on Sunday morning won't be determined by those qualities - they will be there as human battering rams, tasked with thundering into the thickest traffic with the goal of getting over the gainline.
That's life as an international second-five these days. The natural ball-payers can't get a look in. Someone like Aaron Mauger, clever and visionary, a class distributor and decision-maker, wouldn't get near the All Black No12 jersey if he was in his prime today.
Going further back, even the likes of Walter Little wouldn't be comfortable in this world of direct running behemoths, and further back still, the likes of Mike Gibson, Ian McGeechan and Jo Maso would be crushed in an instant - the game almost mocking their craft and guile.
The emphasis on size and physicality is not everyone's cup of tea, but it is the reason Nonu and Roberts are respectively rated number one and number two in the world: this is the best No12 in the Southern Hemisphere taking on the best in the North.
"[Roberts] is a big player and I guess I have played him four times in the last few years and every year it is a bit different," says Nonu. "You know you are going to have big collisions and you know you have to manoeuvre yourself really. It can become a one-on-one contest but it is all about the team in the context of the game-plan. But I'm sure our paths will cross."
Everyone will feel it when they do. In a career that has spanned almost a decade, not only has Nonu rarely been injured, he's rarely come off second best in any collision. Even the likes of Jerry Collins, Richie McCaw, Schalk Burger and Jerome Kaino have been sat down on occasion, left rattled and dazed after a major collision.
Nonu says he has been too, although he can't remember any specific occasions.
"I have come off second best a few times," he reckons. "I try not to run into the forwards as a general rule. But how we are trying to play - we want to win the gainline.
"I guess the midfield is clustered and a lot of that depends on the lineouts really. [It's clustered] if you have five-man lineouts."
Roberts, who is hoping to graduate from medical school next year, has the advantage of fully understanding the consequences of not being prepared or accurate enough in the midfield exchanges.
The chances of serious physical damage being inflicted increase markedly when the tackling technique is poor or if the defender doesn't get into the right position to make the initial hit. There's the added problem that the defensive line will be broken and there is a feeling these days that any break through the midfield should be converted into a try.
Wales have seen how the All Blacks, through a combination of trickery, brute strength, deception, accuracy and creativity, have opened up opposition defences this year and on this tour specifically.
Roberts and his midfield partner Jonathan Davies have, therefore, spent much of the build-up analysing the All Blacks' offensive patterns and honing their defensive response.
"The tempo with which they play the game and the way Carter is a threat whether he is running, kicking or passing" is the biggest threat, Roberts says.
"You switch off for one minute and you will be standing under the posts. They are a team that loves to play on the front foot and the accuracy with which their backline plays is pretty special, their wings are always involved, in the picture and their decoy lines are effective and they are big men.
"If your body position is poor for a split second they will exploit that. That is one thing we have worked on all week - getting our body profiles right, making sure we are an effective defensive unit. You need to stop these guys from playing and meet them on the gainline - that's the key to the game."
If nothing else, the game against Samoa will have prepared Roberts for the physical onslaught that awaits. The Samoans came with the same plan to own the collisions and win the gainline and former Blues fullback Paul Williams was asked to confront Roberts at No12. The two hit each other head-on several times until Williams had to leave the game after 55 minutes with a broken cheekbone.
Nonu is expecting a similarly ferocious afternoon and not for one second is he buying the notion of Wales being a dead duck after five consecutive defeats.
"I think Wales have come into their own in the last four or five years," he says. "The influence of Gatland has done wonders for their side and they have got a great defence. They showed last year they could be the best in the world."
- Gregor Paul of the Herald on Sunday