At the time it didn't look much. Two big blokes – at 6ft 4in and 17st just above average for the centre nowadays – collided at the start of an international. It happens all the time. But then they replayed it on the big screen in slow motion, and the clash of Stirling Mortlock's and Jamie Robert's heads made 74,000 spectators at the ­Millennium Stadium wince.

Mortlock, the veteran captain of Australia, stayed down, knocked out cold. "Easiest cap I ever earned," he later told Roberts when they swapped their clean shirts. There were just two minutes on the clock. Roberts, the young convert to the inside-centre position from full-back or wing, groggily got back to his feet and played on for 10 minutes.

"It was the adrenaline," he says. As a medical student, he should know. Roberts is not consumed by rugby outside work hours with the Cardiff Blues and Wales, yet he is intrigued by his case notes.

Adrenaline still pumping, he was numb at the point of contact; his forehead against Mortlock's skull. Roberts even managed to make a sizeable contribution to the first Wales try, a run sandwiched between Shane Williams' initial break and the winger's finish. And still he found fault with himself for not making it all the way to the line. "Perhaps I should have handed Drew Mitchell off ..." he muses.

Seven weeks later and two days before he reclaims his place in the Blues' starting line-up against Gloucester in the Heineken Cup, he may well be forgiven. He had, after all, played with a fractured skull.

He runs a finger down a brow that bears no mark of the injury."I suppose," he concedes. "It's all a bit hazy. I do remember that suddenly the adrenaline wore off and I had this blinding headache. I had to go off. Not that that was the end of it. Down in the changing room, I started to have a nose bleed. I thought to myself that I hadn't hit my nose, so I was a bit worried by that. And then, when I stood up I heard this, well, it wasn't a grinding, but a sort of squeaking in my head. Something was definitely not right."

The fissure that went down his forehead and behind his eye socket has healed. The noises in his head have stopped and he is back in his new position, wearing the No12. He has doubts, not about the injury, but about the role. "I've only played about 15 games at inside-centre. But if the national coach [Warren Gatland] asks you to change roles, you can't say no."

Roberts was outstanding at 12 in the second Test against South Africa in Pretoria last June. Powerful and thoughtful, he opened up a whole avenue of opportunities for himself and his team. It appears doubts are not necessarily bad for Jamie Roberts. "I remember when I won my first cap, against Scotland in the last Six Nations, I was worried beforehand about having the right to be on the international field. All those great players out there.

"And then after about 20 minutes, I thought, 'I can do something here. Add something.' That felt good."

The player who made a stadium wince, and who can worry for Wales, plays tomorrow at Kingsholm. Those two hindrances may be for others now that Roberts is back in the collision business.