At half past the hour on Tuesday, with the ball nowhere near him and no opponent inside three yards, William Saliba pulled up while jogging back into his own half, sat down on the Bernabéu turf and reached for his lower back. He did not stop at the bench on his way off. Television caught him walking straight past it and down the tunnel, and what he told the French medical staff before he went was reported afterwards in four words: "My back is gone." Maxence Lacroix came on in his place, France reshuffled a defence built around him, and Arsenal supporters watched a World Cup semi-final turn, for a few minutes, into something closer to a hospital bulletin.

It would be tidier to call this a shock. It was not one. Reports out of the French camp before the tournament even began said Saliba had been playing through discomfort in that same area since the closing weeks of Arsenal's title-winning season, discomfort he managed rather than solved. Well enough to get through 43 games and more than 3,700 minutes across the league and Europe, but not well enough to make it disappear. He missed France's group match against Norway on medical advice. He missed training sessions he'd normally have done in his sleep, following what was described as an individualised recovery plan. None of that is the profile of a player who tweaked something on a Tuesday afternoon. It is the profile of a problem that has been managed, carefully and for months, right up until the moment management wasn't enough.

The specifics matter here, because vague language is how clubs get away with saying nothing. Analysis of the injury circulating since points to spondylolysis: a stress reaction, potentially a stress fracture, in the pars interarticularis, the small bridge of bone at the back of each vertebra. It is the sort of injury that comes not from one bad tackle but from thousands of repetitions of exactly the movement a modern centre-back does for a living, extension and flexion and rotation under load, over and over, for a club that has asked him to play almost every minute of almost every game for three seasons running. Left to progress, the same report notes, it can develop into spondylolisthesis, where the vertebra itself starts to slip and the disc gets involved. Arsenal's own caution with him this season, the reported four-game absence and the careful staging of his return, reads in that light less like fussiness and more like a club that already knew roughly what it was dealing with and was trying to buy time.

The case for leaving well alone is not a stupid one, and it deserves saying properly rather than waved through. Saliba is not merely good; he is a specific kind of good that the squad is built around, comfortable on the ball in ways that let Arsenal play out under pressure and control matches other teams would simply survive. His most obvious understudy, Rob Holding, offers close to the opposite set of strengths, solid and committed and not remotely the same footballer, which is precisely why his four-game absence this season coincided with a dip in form that had nothing mysterious about it. Buying a defender in a hurry, mid-window, to plug a gap that might not even be permanent carries its own risk: a player bought for a crisis rarely settles like a player bought for a plan, and Arsenal have spent enough of the last decade unpicking panic signings to know the difference. If the club's medical staff believe conservative management is working, that view should carry weight.

They see the scans. We see the highlights.

But conservative management was already the policy in May, and here we are in July with the player walking down a tunnel in Madrid saying his own back had gone. That is not evidence the plan failed exactly. Stress reactions in bone do sometimes heal on rest and still recur under load, which is a horrible sentence to have to write about a twenty-five-year-old. It is evidence that hoping for the best has already been tried, for months, by people considerably more qualified than anyone writing a column about it. A repetitive-load injury does not become less repetitive because the season restarts in August with a full pre-season and fifty-odd fixtures ahead of him. The workload that produced the stress fracture is the same workload Arteta will need from him again inside four weeks.

I've resisted, for several paragraphs now, the obvious pub argument, which is that Arsenal should have addressed this depth problem last summer, before a World Cup ever got the chance to expose it. Let's have it anyway, because it's true and saying it doesn't require hindsight. The imbalance between Saliba's minutes and Holding's minutes was visible in a lineup sheet, not a scan. It just took a stadium in Madrid and four words in a tunnel to make it feel urgent.

The window doesn't close until the first days of September, which gives Arsenal room to do this properly rather than desperately, to sign a centre-back not as a replacement for Saliba but as insurance against the version of this season where he needs eight weeks in October that nobody currently has budgeted for. The alternative is trusting that a stress fracture in a young man's spine will simply behave itself for the seventieth consecutive week of first-team football. It might. It has, until Tuesday.