IF Test rugby is a 23-man game these days, as coaches always insist, what is the size of squad needed for a tour? How many players must you be able to call on – not just the initial selections, that is, but the total number used – to give yourself the best chance of winning?

Coaches emphasise the quality of the bench as well as the starting 15 for a very good reason: it is rare indeed for a match to get to the end without there having been a medical need for substitutes, never mind a tactical desire. The days when Scotland could win a Grand Slam after using only one replacement in four matches, as they did in 1984, seem like centuries ago, so much has the game changed.

If fortune is in your favour, there are some matches where you can get away with having the odd player on the bench you would rather not have to call on: someone who is only there because better, more experienced players are unavailable for one reason or another; someone who can just about toddle on for the last 10 minutes or so if the match is won or lost by then.

But in a tournament as attritional as the Six Nations, such fortune does not last, and by the end of the championship it is invariably clear which squads have greater strength in depth. 

So too with tours. Long tours such as the Lions’ upcoming series in New Zealand obviously highlight the importance of quality replacements, but so too do shorter ventures such as Scotland’s forthcoming three-Test 
trip to Singapore, Australia and Fiji. Accidents and injuries can happen 
in training, as we were reminded this week, and at the end of a long season some knocks can perhaps prove more damaging than they would in the autumn.

Richie Gray, for example, has pulled out of the Scotland tour because of a back complaint which he had been nursing, in addition to taking ill, while Sam Hidalgo-Clyne has had to withdraw after suffering a hamstring strain. The Lions camp, meanwhile, has lost England forward Billy Vunipola because of a shoulder injury. These losses are highly unlikely to be the last for either tour party, and the response to them by the respective coaches, Scotland’s Gregor Townsend and the Lions’ Warren Gatland, have given some indication of how they plan to conduct their tours. 

As he was third in the scrum-halves’ pecking order behind Glasgow’s Ali Price and Henry Pyrgos, Hidalgo-Clyne was always going to be one of the 
easier squad members to replace. When he originally announced his squad, Townsend said that all three Edinburgh No.9s – Sean Kennedy 
and Nathan Fowles being the other two – had been in a close fight to join the Warriors pair. Calling up Kennedy, then, is likely to have been a fairly easy decision.

Presuming Price and Pyrgos stay fit and in form, and Kennedy acquits himself well if called upon, the biggest loss will be to Hidalgo-Clyne himself. After playing so well in the 2014-15 season, he was taken to the World Cup but barely saw any action, and since then has struggled for form and a long run of starts. Getting involved again with the national team could have been just the thing the 23-year-old needed to rekindle the spark in his game.

Gray, by contrast, may well find a rest beneficial in the long run. It is often a misnomer to call an injury a blessing in disguise, but the demands on lock forwards are so heavy these days that an extra rest is often profitable, for all that the player in question finds it deeply frustrating 
at the time.

Scotland, of course, will not find the lock’s loss beneficial at all. Not only is Gray a key player in his own right, his partnership with younger brother Jonny has blossomed over the past year and would have been a vital element of the battle for dominance up front against Italy, Australia and Fiji. Townsend has not called up a replacement yet, but with only Jonny Gray, Tim Swinson and Ben Toolis to choose from in the second row, an addition is likely.

The Scotland coach named 34 players in his initial squad, while Gatland announced a group of 41 for the Lions. Townsend is likely to have taken charge of nearer 40 by the time his group come home, while Gatland will surely be closer to 50. Both squads will likely be stretched to the limit by the time they reach such numbers, particularly given the quality of the opposition they will face in the Wallabies and the All Blacks. 

Indeed, it is arguable that the Lions are already close to that limit, given how vital Vunipola could have been to their campaign. Gatland’s selection of James Haskell as a replacement for his England back-row colleague is a debatable decision, but in truth there was probably no one who could have come close to emulating Vunipola’s drive and dynamism. 

Even when you are pooling the rugby resources of four separate 
teams, some individuals are close to being irreplaceable.