Mako Vunipola in line for debut against Fiji after Graham Rowntree says 'he turns me on'

England coach predicts uncapped 21-year-old will make big impact as squad is named for the autumn internationals

Graham Rowntree took a temporary turn as the England coaching team's top-of-the-bill attraction today – the head coach, Stuart Lancaster, was out of town, attending a family funeral – and predictably enough, the cauliflower-eared prop of yesteryear spent much of his time talking about a front-row forward: Mako Vunipola. The uncapped 21-year-old loose head from Saracens has leapfrogged Nathan Catt of Bath and Matt Mullan of Worcester to one of four spare places in the 32-man party for next month's Tests against the pick of the southern hemisphere.

"I'm looking for a fault in Mako's game and I can't find one at the moment," said the red-rose pack strategist. "He's got himself fit, which is a big step forward, and he's put together a string of performances in the Premiership and Heineken Cup this season that we just can't ignore. I've been watching his development for a long time now and I like his commitment to every aspect of the game. He carries the ball dynamically, but he still loves to scrummage – and you know that turns me on."

Leaving aside Rowntree's personal predilections, together with the mind-stretching notion that someone weighing 20st could conceivably leapfrog anybody, there is clear excitement throughout the England camp at Vunipola's sudden arrival in the senior squad. Born in New Zealand to Tongan parents – his father, Fe'ao, captained the island team and played in both the 1995 and 1999 World Cups, while his brother Billy plays No 8 for Wasps – the prop has developed rapidly since leaving Bristol for Saracens in the summer of last year.

"Mako has grown up a lot," said Rowntree's coaching colleague Andy Farrell, who was working with Saracens when Vunipola joined the club and collaborated closely with him for the lion's share of last season. "He really got stuck into his fitness, and also into the way he lived his life off the field. He'll definitely add to what we're doing with England."

Vunipola fills the gap, considerable in every sense, left by the London Irish forward Alex Corbisiero, who did so much to drive England forward at close quarters in the 2012 Six Nations but has been struggling for fitness for the best part of six months. The Northampton outside back Ben Foden, the Bath hooker Rob Webber and the Leicester flanker Tom Croft are also off the roster for the autumn internationals for a variety of orthopaedic reasons. As expected, they have been replaced by Ugo Monye of Harlequins, Tom Youngs of Leicester and James Haskell of Wasps.

Of the four additions to the squad, only Monye and Haskell have reasonable shouts of starting against Fiji, the first of this season's visitors to Twickenham, in 16 days' time. But Youngs, another uncapped forward, will certainly be on the bench as understudy to the outstanding Dylan Hartley of Northampton while Vunipola can also expect to be involved, now that the International Rugby Board has sanctioned the use of an extra front-rower in an effort to minimise the risk of uncontested scrums – rugby's version of the ultimate passion-killer.

According to Rowntree, incumbency will be one of the principal considerations when he, Lancaster, Farrell and Mike Catt decide on the starting combination for the Fiji Test (which will be followed in consecutive weeks by meetings with Australia, South Africa and New Zealand). "It will certainly count," he said. "When someone puts in a good performance in an England shirt, especially in a difficult match in a tough environment, that's credit in the bank with me."

On this basis, Haskell has a chance of beating Tom Johnson of Exeter to a place in the starting back row: he made 28 tackles, a significant number by anyone's standards, in the drawn Test with the Springboks in Port Elizabeth in June – the flanker's first appearance for his country since last year's World Cup. It would also appear to put Danny Care of Harlequins ahead of Ben Youngs and Lee Dickson, his fiercely competitive rivals for the scrum-half berth, and give the Saracens' full-back Alex Goode an edge over another Quin in Mike Brown.

Meanwhile, the Wallabies have named a strong squad – rather stronger than expected, if truth be told – for their end-of-term trek to Europe, which also features Tests in France, Italy and Wales. The open-side flanker David Pocock, the hooker Stephen Moore and the goal-kicking midfielder Berrick Barnes will all make the trip after quick recoveries from worrying fitness issues. There will be no Quade Cooper or James O'Connor, for behavioural and injury reasons respectively, while the suspended back-rower Scott Higginbotham has been omitted because he would have been ineligible for the first two games, but under the leadership of the magnificent lock Nathan Sharpe they will at least be recognisable as the side standing second in the world rankings.

Fab four: Who are new additions to squad for autumn matches?

Ugo Monye

The experienced Harlequins finisher has not played for England since the 2010 Calcutta Cup match in Scotland, but with his club playing at a tempo that suits his game perfectly, the 29-year-old has rediscovered the comforts of confidence.

Mako Vunipola

His progress through the England age-group sides brought him to the attention of the wider coaching community and once he left struggling Bristol for Saracens, a real power in the land, his rise up the pecking order was assured.

Tom Youngs

Brother of the England scrum-half Ben, he started life as a centre but as always a "brick outhouse", as Graham Rowntree put it yesterday. Combining a midfielder's footballing skills with close-quarter strength, he is a thoroughly modern hooker.

James Haskell

A substantial figure physically and an unusually big character, the flanker put his international career at risk by heading off to France, Japan and New Zealand either side of last year's World Cup. He has, however, returned wiser, as well as older.