Even by Test match standards an awful lot has been said and written about Sunday's head to head between England and Wales at Twickenham, but some words of Stuart Lancaster, reported last week, seem to have slipped under the radar. Announcing that Billy Vunipola would miss the final two games of the Six Nations, the England coach sought to reduce the impact of losing their No8 by saying that in Ben Morgan he had a like-for-like replacement. I don't think so.

In fact, I'd say that the loss of the 21-year-old Saracen, a man with only eight caps and less than a year's Test rugby behind him, could change the whole dynamic of what England have become.

This isn't to say that Morgan is not a fine player. In the past, notably in Paris two seasons ago and in the game following, when he was man of the match against Ireland, he excelled and has not let England down one jot when he has come on from the replacements' bench this Six Nations. But a Vunipola he isn't and part of the key to understanding what Morgan is, is to know about the six-figure mistake Gloucester made when he slipped through their fingers.

Much has been written about Morgan being spotted by the Scarlets and honed into the mobile back-row he is, but little is known about the frustrated wing that wears the England No8 shirt on Sunday afternoon.

Morgan comes from Dursley, just 12 miles down the A4135 from Kingsholm. For a short time he was in the academy, playing on the wing at Cinderford ... but only a short time. Four stone overweight, Morgan was shown the door. It was an embarrassment kept quiet when he was bought back from the Scarlets, but it's a little bit of history which should help understand the way Morgan plays, as against the approach Vunipola has brought to England and which, I say, has benefited immensely those immediately around him.

Put simply, Vunipola has that precious ability to put England on the front foot and to do it from a standing start, whereas Morgan benefits from momentum. He does best getting the ball on the move; Vunipola does damage even when receiving the ball standing still.

Think of the times Vunipola has launched his near-20 stone this Six Nations, particularly from the base of a ruck or scrum, then look around at those who benefit from the way he wins his yards. Immediately you look at those either side of him in the back row – Tom Wood and Chris Robshaw.

A year ago, after England had been so badly beaten in Cardiff, the conversation was all about Robshaw and his inability to play as a conventional No7 – the fetcher, the openside flanker as happy on the ground as linking with his midfield. At Cardiff he patently wasn't one, whereas Wales had Sam Warburton and Justin Tipuric, and England suffered horribly from the turnovers.

Wood looked almost as unhappy playing No8, but suddenly the arrival of Vunipola has given the England back row the balance of a Swiss timepiece. We're not talking about Robshaw not being a No7 any longer, but part of a world-class back five in the pack. Much of the reason is that Vunipola has made life that much easier for Wood and his captain because their No8 puts them on the front foot where their enormous work-rate does so much good.

But England's two flankers aren't the major beneficiaries. If you analyse the improvement in the play of the scrum-half Danny Care, and the game management of his half-back partner, Owen Farrell, you can see that they are also finding life a lot easier with the big man making those dynamic yards close to where the action is.

That's where he likes to be and that's where he does so much good work, constantly getting involved in the phases. No way will you find Vunipola floating around out on the wing like the great All Black No8, and player of the year, Kieran Reid. However, that's where you might find Morgan.

Like Vunipola, the Gloucester No8 might be used off a five-man lineout or standing as first receiver, but Morgan is happier out of the heavy traffic. Whereas Vunipola can hurt defences, making the hard yards right under opposition nose, Morgan might make more yards, but they tend to be wider out. He's happiest and most effective when you don't see him coming. Like for like? Not really.

It will be interesting to see how England and Lancaster cope with the change forced by Vunipola's damaged ankle. They have come a long way since Cardiff 2013, but Warren Gatland and Shaun Edwards will have seen what England, Care and Farrell can do with Vunipola around and will almost certainly use the likes of Warburton, Dan Lydiate and Toby Faletau to keep the area around the breakdown and ruck as static as possible.

If they do, England's improving half-backs will have to find another way of playing or face a hard day at the office.