Cape Town – Don’t they say wildfires are a good thing for the landscape in many ways?

If so, then you might say a blaze has officially been lit by head coach Rassie Erasmus beneath Franco Mostert and Pieter-Steph du Toit in a captivating duel – possibly right up to and into RWC 2019 – for the Springbok No 5 jersey.

Versatile, proven Lood de Jager, currently injured, cannot be ruled out of that particular positional battle further up the line, but for the moment just the two fit specimens are engaged in a fierce tussle for the berth.

In the very debatable, lone change to the Bok starting XV for Saturday’s Rugby Championship Test against Argentina in Mendoza (21:10 start), Du Toit drops down to the bench – where it looks far more likely that he covers loose forward, into the bargain – to make room for Mostert in the second row.

That is despite Du Toit, in general play, having had a more than decent Test match (at least by this writer’s estimation) in last weekend’s 34-21 triumph over the same foes in Durban.

But in Thursday’s official team-announcement release from the Bok camp, Erasmus was significantly quoted as saying: “He’s (Mostert) been our first-choice No 5 lock and that’s why he is starting on Saturday.”

Whatever your own thoughts on the matter, hand it to Erasmus … that’s about as direct as you can get with the media. No bull-dust.

He reminded simultaneously that Mostert had been purposely rested for the Kings Park Test considering his more extended workload in Super Rugby this year; the Lions reached the final again.

The pro-Du Toit club might be inclined to argue that their man did enough – and then some? -- in Durban to warrant retaining his berth, and that Erasmus might have adhered more closely to the principle of individuals “playing themselves out” of a particular sports team.

In broadest sense, the Stormers player didn’t seem to do that last Saturday, his grunt-laden alliance with the amazingly, rapidly resurgent Eben Etzebeth, an old ally in Cape Town, being a hallmark of a dominant Bok pack showing as a whole.

It is also worth bringing to light that Du Toit had – admittedly a slightly subjective statement again – seemingly been one of the shinier lights at blindside flank in each of the last two Tests of the June series against England.

So whether he deserves slipping down to the substitutes this Saturday, against such a backdrop, is a healthy topic for discussion.

Then again, Erasmus has been fairly clear-cut, logical -- and successful -- in much of what he has done selection-wise since taking the demanding Bok reins, and has come across as a sound man-manager, into the bargain.

And if you removed completely from your mind last weekend’s Championship opener, when Mostert was purposely idle, you could quibble even less with the now former Lions workhorse’s inclusion here: he had formed a compelling lock combination with RG Snyman (back on the bench himself this Saturday) in the victorious England series.

The recent Gloucester acquisition is undoubtedly one of those “100 percent effort” type of tight forwards -- on a consistent basis, whether for club or country -- and Erasmus may also feel that he is superior at this stage to Du Toit for lineout calling and reading.

The department was one of the relatively few to go notably awry in Durban, and the coach was entitled to feel that some sort of freshening or jacking-up of standards was required there for what might be an altogether stiffer Mendoza tussle.

South Africa will sacrifice a bit in pure beef terms – this in the land that prides itself on its red meat, remember – this weekend, as Mostert tips the scales a few kilograms short of Du Toit, but his synergy at throw-in time with old Lions team-mate and hooker Malcolm Marx will have helped, no doubt, tilt his selection.

Expect this much on Saturday, now that Erasmus has laid his cards, perhaps shrewdly, so firmly on the table when it comes to No 5 in the Bok line-up: Mostert will want to provide nothing less than another frenzied, full-blooded shift, knowing that he has a quality rival still breathing down his neck.

As for Du Toit, when unleashed off the splinters – as he surely will be, regardless of in what capacity – the blond competitor should be snorting with constructive rage, too.

If so it would be a carbon copy of last year, in the equivalent fixture in Salta, when he also appeared as a sub and was an urgent, aggressive contributor to the 41-23 victory (one of the better ones, really, in the Allister Coetzee tenure as coach).

The starting locks that occasionally ill-tempered day?

You guessed it … Messrs Etzebeth and Mostert.

*Follow our chief writer on Twitter: @RobHouwing