The loss of Leigh Halfpenny for Wales before the World Cup is grievous, the ramifications the same as if England had been left to make do without Jonny Wilkinson in 2003. If Warren Gatland has to say that the period of mourning sickness will last no longer than the time it took to read the medical report on the state of the full-back’s right knee, Wales’s head coach has to plan for life without his probably most valuable player.

Gatland has cover at full-back in the form of Liam Williams, who has not played since last season because of a foot injury, and the inexperienced Hallam Amos but Halfpenny is a player, like Wilkinson, who is the ultimate professional, someone who sticks to a gameplan and invariably executes it perfectly, an exemplar in training and able to kick goals on the most demanding of occasions.

Sport reflects life in its random cruelty. Gatland has been criticised for playing Halfpenny against Italy even though his right knee was strapped, as it had been the previous week against Ireland, when nobody demurred. The injury he suffered was a freak accident, his foot caught in the turf as he tried to adjust to an unexpected bounce of the ball. Had there been any doubt about his fitness, he would not have played.

Wales have to adjust, although they will go into the tournament lacking a kicker capable of landing penalties from his own half, just as New Zealand had to adjust in 2011 when they lost their outside-half Dan Carter during the group stage. Italy have fears over their captain, Sergio Parisse, who picked up a leg injury against Wales; without him, their small chance of making the last eight would all but disappear.

Wales have also lost their scrum-half Rhys Webb, a player of less experience than Halfpenny but one who was central to their policy of achieving a ball in play time of more than 40 minutes through his ability to get the ball away quickly from the breakdown and his eye for a gap around the fringes. A favoured ploy in opposition territory is to use possession from the top of the lineout to get the centre Jamie Roberts over the gain line. Every second counts and the speed of Webb’s pass contrasts with that of the more laboured Mike Phillips, who had come to narrow the point of attacks.

The England backs coach, Andy Farrell, said this week that his squad would be able to overcome seamlessly the loss of a player of Halfpenny’s value. He was duty-bound to say that but one reason England went into the final warm-up match against Ireland last weekend in a state of uncertainty in some positions was an inability to fill the holes left by two suspended players, the hooker Dylan Hartley and the centre Manu Tuilagi. Such is the polish that Hartley brings to the set pieces that he will surely be recalled to the squad if one of the three hookers suffers an injury after the end of next week.

Farrell also said that Gatland would use the loss of two of his backs, which followed the injury to the Lions centre Jonathan Davies at the end of last season, to give Wales a backs-to-the-wall mentality, turning adversity into advantage. When Sam Warburton was sent off 20 minutes into the 2011 World Cup semi-final against France, which followed the loss of the tighthead prop Adam Jones to injury moments earlier, Wales found strength from somewhere and would have won had Halfpenny’s late long-range penalty – it was the last match that he was used by his country as a kicker from distance rather than from everywhere – not fallen a metre short.

Wales will still be hard to beat but without Halfpenny they will find it harder to defeat England and Australia – and not only because of his high goal-kicking percentage. He is one of those players who is immaculate in everything he does, a coach’s dream, conspicuous by his absence. Williams is more assertive, especially with the ball in hand, but more prone to error and it is Halfpenny’s constancy, his ability to be in the right place at the right time, that Wales will miss, as would any team.

One of the reasons Gatland played a stronger side against Italy than he had been considering was his desire to rest most of his frontline players from Wales’s opening game against Uruguay, knowing that some of them would have to play three in a row against England, Fiji and the Wallabies. He felt the warm-up victory in Dublin had shown a layer of rust it would take another game to remove. No matter how assiduous the planning, however, injuries are an unforeseen element to coaching and while Wales on paper have a stronger first-choice team than England, the men in white have far deeper cover.

The World Cup is likely to be decided by strength in reserve, such is the attritional nature of international rugby. If the qualifiers from England’s group will have an advantage going into the quarter-finals of being match hardened, they will be fortunate if at that point they have not had to call for reinforcements. Wales have the disadvantage of already having had a second layer exposed.

Stuart Lancaster has had such a torrid two years in terms of injuries that the England head coach will have been considerably relieved to have come through his side’s three warm-up matches with no fitness issues. His dilemma is one of selection, not least his bench. With Brad Barritt covering 13 from 12 and Anthony Watson and Mike Brown covering each other’s positions, Sam Burgess, an impact player as Ian Madigan found out last weekend, is an option even though he does not provide versatility; England will not consider him as a back-rower. It is a considerably better issue to have to wrestle with than how to replace Halfpenny.

• This is an extract taken from the Breakdown, the Guardian’s weekly rugby union email. To subscribe, just visit this page, find ‘The Breakdown’ and follow the instructions.