Thirty minutes into a World Cup semi-final in Dallas, on 14 July, William Saliba went down again. He had been carrying the back problem through the tournament for weeks, playing on it because that is what defenders who function as the spine of a title-winning side do, and then Spain took the ball away from him permanently. He missed the bronze medal match against England. He came home to London, saw a specialist, and Arsenal confirmed what everyone at the club already suspected: no surgery, but an extended period out, a managed recovery programme with no return date attached to it.

Fifty appearances last season. A Premier League title after twenty-two years without one. A run to the Champions League final. Saliba wasn't the top scorer in that story and nobody expects him to be — his value was in the thing that doesn't show up on the highlights reel, the back four organising itself around a twenty-five-year-old who reads the game a half-second before it happens. Arsenal now begin the defence of that title without him, against Manchester City in the Community Shield on 16 August, then at home to promoted Coventry five days later. Nobody at the club is calling this a crisis. They're calling it a rehabilitation programme. Those are different sentences describing the same hole.

The case for calm is not nothing, and it deserves saying properly before it gets waved away. Ben White is back in training after the knee injury that ended his season in May. Cristhian Mosquera and Riccardo Calafiori are both back for pre-season and can play centre-back if required. Jurrien Timber, who missed the World Cup with a groin problem, says he's hopefully back soon — hedged, but not alarming. And the underlying argument, the one Arteta himself would make in a press conference without blinking, is that a squad which won the league on the back of a genuinely excellent defensive record has earned the benefit of the doubt for six or eight weeks of one player's absence. Good sides absorb bad luck. That's supposed to be the whole point of building one.

Here's where the maths stops working for me. White is a right-back who can play centre-half in emergencies, not a like-for-like replacement for Saliba's specific job, which is covering the space in behind Gabriel while also stepping into midfield with the ball at his feet like a man auditioning for a different position entirely. Mosquera has barely a season of senior football in him. Calafiori is left-footed and left-sided by instinct, and asking him to slot into Saliba's side of the pairing changes the whole shape of how Arsenal build from the back. Timber is "hopefully" fit, which in football English means not fit. Depth on a team sheet is not depth on a pitch. Arsenal have bodies; they do not have Saliba's job covered by someone who does it the way he does it.

Which makes the summer's business worth a second look. Nobody at the club has said out loud that centre-back reinforcement wasn't the priority, but you can read a transfer window the same way you read a back four — by where the money and the urgency actually went, not by what the statement says. This summer has been about adding goals. That's a defensible strategy if your first-choice defence stays healthy. It becomes a live problem the moment it doesn't, and Saliba being unavailable for the opening two months of a title defence is exactly the moment. You don't get to plan for a squad's greatest strength being permanently available and then act surprised when it isn't.

The bloke behind me put it simpler than any of that during the second half of a pre-season friendly last week, watching Mosquera get turned by a striker who had no business winning that duel: "That's not a back-up plan, that's a prayer." He wasn't being cruel about the lad. He was being accurate about the gap between what the club is asking a twenty-one-year-old to be and what he currently is.

None of this means panic, and none of this means the title is gone before August has properly started. It means Arteta has to coach differently for a while — drop the defensive line deeper against City's runners, ask the midfield to protect the back four rather than trust it to protect itself, accept a version of Arsenal that wins some of these early games ugly rather than the one that strolled to a title playing through thirds like it was easy. That's the tax the plan sometimes charges you, and paying it isn't a betrayal of the project. Pretending you don't owe it is.

Saliba comes back when he comes back. What Arsenal do between now and then is the actual test of whether last season was a foundation or a fluke wearing a nice badge.