There are only eight more tests before the World Cup and Williams needs opportunities to get comfortable with the All Blacks game plan.
That's a handy line for All Black coach Steve Hansen to trot out. There was, however, a more immediate reason for picking Williams to play at Twickenham - he is explosive, gifted and uninhibited. "We looked at Malakai [Fekitoa] and he's had a massive season so we thought that having a breather for a few weeks wouldn't do him any harm," says Hansen on the reasons why Williams was picked.
For England, the arrival of Rokoduguni from virtual unknown to England debut in the space of a few months is all about x-factor.
For all the improvements England have made in the past two years, they are still short of genuine game-breakers: men with fast feet, evasive qualities and the confidence to back themselves one-on-one.
Rokoduguni has scored tries and pulled off a couple of steps way beyond the skill level of his Premiership peers and England coach Stuart Lancaster wants that unpredictability and spontaneity.
He has seen what Billy and Mako Vunipola and Manu Tuilagi - other players with a Pacific Island background - have done for England and he wants a bit of that explosive power and creativity on the wing.
It's a little hard to comprehend that in a corner of southwest London, the influence of tiny islands virtually cast adrift in the Pacific Ocean, will be enormous.
There will be 30 players on the field and yet the chances are high that the players with Pacific Island heritage will have a major influence.
Williams could be the biggest point of difference - exposing England's defensive vulnerability in the midfield. But Julian Savea hasn't had a quiet game against England: in five tests against them he has scored eight tries.
They haven't yet found a way to deal with his pace, agility and timing. Then there is Jerome Kaino - restored to the starting XV after various injury niggles. He's eager and ever willing to make his presence felt.
Charlie Faumuina will come off the bench and add something worth watching, as will Patrick Tuipulotu.
England will ask Vunipola to charge at the All Blacks for as long as he can - knowing that he caused a few problems last year. At nearly 130kg, he takes a bit of stopping.
As for Rokoduguni, no one quite knows what they are going to get on debut but Lancaster's great hope is that the Fijian will bamboozle with his step and bump a few defenders off. "It is good to see the little nations making big waves overseas," says Kaino, who's from American Samoa.
It just shows how much those little nations love rugby and anywhere they go in the world they are able to make an impact.
Rugby just can't get enough of Pasifika.
Fortress Twickenham an easy bastion for foes to breach
Fortress Twickenham might be more wishful thinking than anything else.
England are desperate to turn their magnificent home ground into one of the most intimidating venues in the world game and convince those who visit that they should abandon all hope.
They face a bit of a sales job. In the past 10 years, England have a 62 per cent win record at their spiritual home. They have played 56 tests and won 35.
That's hardly an imposing stat - it's barely a respectable stat and doesn't stand comparison with the All Blacks at Eden Park.
In that same time, the All Blacks have played 23 Eden Park tests and haven't lost. It was 20 years this July since they lost a test at Eden Park and while there has been the occasional close shave, the All Blacks have mostly destroyed teams in Auckland.
That's a proper fortress. Eden Park is a ground that intimidates opponents. England themselves appeared to take fright this year when they were still in business in the final 10 minutes.
At 15-all the game could have been theirs, but the control, poise and confidence they had shown in the first 70 minutes left them in the last 10.
It did appear as if they looked around, realised where they were and talked themselves out of the victory.
They want Twickenham to have that same aura - but it's not possible to create a fear factor with noise and branding alone.
The ground is a cathedral and the noise when England are in full flight is deafening. But visitors know they have to defend and endure bursts of momentum and pressure in the same way they would at any venue against any good opponent.
Without a consistent record of success, England can't expect to put doubt in opposition minds; the All Blacks, for instance, know the pressure will swing back in their favour at some stage.
And they know this because in the past 10 years they have played at Twickenham seven times and won six. Where is the fear factor in that?
England desperately want to believe they have a place no one else wants to go. South Africa have Ellis Park and New Zealand Eden Park - so part of that desire is about keeping up with the Joneses.
But a bigger part is about creating perception before the World Cup. Home advantage has been important at previous tournaments. New Zealand made it work for them in 1987 and 2011 and South Africa in 1995.
England are scheduled to play three of their four pool games at Twickenham, and all of their knock-out games if they make it that far. To have everyone view them as invincible there before the tournament would be rather handy.
A loss to the All Blacks tomorrow morning would put doubt in England's players' minds about their vulnerability at home. It won't be great for them to know that in the last eight tests against New Zealand at Twickenham, they have lost seven.
With South Africa and Australia coming, England could lose three in a row at their so-called fortress and then what?
As well, their overall record under coach Stuart Lancaster won't hold up too well if the defeats continue; he's been coach for 30 tests and won 18.
