'You don't add new confusions to players in the two or three days leading up to a Test match,' Laurie Mains, the All Black coach, cautioned. But with or without Fox's assistance, Ellis could be excused for feeling fairly confused already.
One season on the wing for Otago, another at outside-centre - and, lo and behold, he is suddenly back in the schoolboy position from which the old All Black flanker Jock Hobbs converted him when Hobbs was coaching New Zealand Universities. This will be only Ellis's fifth first-class match in his original position.
Perhaps it was us who were confused, because in the event it was too easy by half to expect Stephen Bachop - who is, after all, the incumbent Otago outside-half - to sidestep into the vacancy left by Fox's non-availability. Mains himself, at the very first press conference the All Blacks held after their arrival in London five weeks ago, pointed out unprompted that he had two very fine first five-eighths (Kiwi-speak for outside-half) in his squad.
He had given Ellis an inkling of what was to come by suggesting during the New Zealand season that he worked on outside-half (sorry, first-five) skills even though he was playing at centre. Ellis himself knew he was bound to be behind Bachop in the pecking order at the start of the tour, and then Bachop gave a glittering attacking performance when the tour opened against London.
So it scarcely looked favourable, especially while the crew-cut alternative outside-half was conducting his personal re-education in the position. 'Steve was the only specialist first-five chosen but I thought I'd be able to put a bit of pressure on him if I got my game right and picked things up fairly quickly,' he said.
And so it came to pass. Bachop has been unable to stamp his authority on subsequent matches as he did at Twickenham, whereas Ellis has looked the part as an attacker with pace and vision and - critically for Mains - as the sounder man defensively. 'Marc does give the opposition plenty to think about,' the coach said. 'He is a very strong defensive player and clearly against Scotland we are going to need strong defence.'
Specifically, opponents have had to think about four tries and a drop goal, but despite these agreeable individual statistics and the enthusiastic notices he has been receiving, Ellis is not wholly satisfied. 'I haven't fully been on top of my game so far,' he said. 'But there are always new targets to meet and I'm never satisfied with my performance.'
New Zealand have solved the problem of being without Fox the place-kicker with Matthew Cooper. Fox the outside-half has not been such a loss because Mains has always wanted to get out of the kicking routine which Fox did so well but which inevitably circumscribed innovative back play.
Still, Fox's vital importance to the All Blacks over more than six years, tactically and as the scorer of 645 Test points, makes him as hard an act to follow as there has been in rugby. The last time Mains tried to do without him, in 1992, he ended up having to bring him back, and even now Fox has not retired.
'He is a great player, one in a million, so it's certainly a big responsibility taking his place,' Ellis said. 'But at the same time I don't imagine I'll be expected to play a similar game to Grant because I'm a different type of player and we're playing a different type of rugby.
'Perhaps there's not quite the pressure there would have been if I was filling the same role, but it was always going to be difficult for anyone who was succeeding a player who had contributed so much.'
Ellis is a native of the New Zealand capital, Wellington, but like so many others he has come through the illustrious rugby nursery at the University of Otago and has played for the provincial side - initially under Mains's coaching - since 1991. He is on a three-year commerce degree course which, in the way of these things in New Zealand rugby, is being stretched over five years. He has another two years to make up his mind whether to stay in Dunedin.
No one in New Edinburgh - it has its own Princes Street and George Street - ever had any doubt about the young man's exceptional talent and he first won his All Black colours on last year's tour of Australia and South Africa. Fortunes were, to put it politely, mixed: Ellis's six midweek games included a 40-17 defeat by Sydney, which remains the worst in All Blacks history.
It was a lesson learned. 'I'm fairly competitive and the tour was good because it made me hungry to be in the Saturday team,' he said. 'And now that I'm having that chance I need to establish myself. I wasn't content solely to be an All Black; I wanted to be a Test player. The next step is to be a good Test player.'
Today is only a start and by the time Ellis is finally established perhaps his hair will have grown back to respectability. He blames the shaven scalps that he and his Otago team-mate, John Timu, displayed on arrival in the United Kingdom on an excess of a Dunedin brew called Speight's, Pride of the South. 'It was a nasty bet with J T that we would shave our heads if we were both selected for the tour,' he said. 'We had had a Speight's - which is a fantastic beer.'
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