Perhaps this wasn’t the game for making proper judgments. Belgium were in the desperately awkward position of being the other team in the face of a great outpouring of Danish emotion, and for half the game they struggled in that role. But the introduction of Kevin De Bruyne, back after surgery on the facial injury he sustained in the Champions League final, changed everything.

But beyond talk of the difficult emotion of the occasion or of De Bruyne’s brilliance, there are other issues for Belgium. Going forward after half-time, they looked like the side that had scored 40 goals in qualifying. Romelu Lukaku, the isolation of the first half ended, was exceptional. But that first half exposed major concerns about the defence: the foundations are creaking.

This was, inevitably and quite rightly, about Christian Eriksen, about working through a lot of the anxieties of Saturday and about celebrating his survival. And, after that, about Denmark getting a win to put them in a position to reach the last 16.

As they came on to the pitch Belgium had two functions. The first was to be respectful participants in the occasion – and their players will have been affected as well. Lukaku plays with Eriksen at Internazionale, and said: “Chris, stay strong, boy, I love you,” into a camera after scoring against Russia, while Toby Alderweireld and Jan Vertonghen were teammates of Eriksen’s for, respectively, five and six years at Tottenham. Then, from a football point of view, Belgium’s second task was to resist the inevitable adrenal Danish surge.

But that resistance lasted only 99 seconds. The major doubt about Roberto Martínez’s side was always the ageing defence. Vertonghen is 34 and struggling with an ankle injury, while Alderweireld is 32. Between them in the back three, Jason Denayer, replacing Dedryck Boyata, is not Vincent Kompany. He does not offer anything like the same composure or authority, and he played a crucial role in Denmark’s opener. It wasn’t so much his weak pass out from the back that was to blame, although that was bad enough, as the way he compounded the problem by being sucked to the ball in trying to rectify the mistake, allowing Pierre‑Emile Højbjerg to lay in Yussuf Poulsen to score.

Yussuf Poulsen exploits Belgium’s defensive uncertainty to put Denmark ahead.
Yussuf Poulsen exploits Belgium’s defensive uncertainty to put Denmark ahead. Photograph: Hannah McKay/AP

The Danish surge, which could have brought two or three goals in the opening quarter, did eventually diminish, but Belgium’s defensive problems remained. Every time Mikkel Damsgaard and his delicate jinking feet got a run at him, Vertonghen looked uneasy. Eventually, it felt, Damsgaard was going to get clipped in the box or the ball was going to fall kindly for him to get a proper shot off. Belgium were fortunate it never quite did.

Martínez has few defensive options: perhaps Boyata could come in, but he is not the most mobile. The only other centre‑back is the 35-year-old Thomas Vermaelen. With modern sports science age may just be a number, but some numbers are still bigger than others.

Where there is De Bruyne, though, there is hope. As soon as he came on, it was a different game. Lukaku, isolated in the first half, had someone to link to midfield, and within 11 minutes the pair had combined to lay on the equaliser for Thorgan Hazard. The winner, a crisp finish from De Bruyne following a slick move initiated by Lukaku’s run down the right – an intoxicating combination of physical power and deft skill – arrived 16 minutes later.

The arrival of Eden Hazard and Axel Witsel from the bench, both recovering from injuries, provided further impetus after the equaliser. Witsel’s presence at the back of the midfield had been particularly missed. And yet still there were Danish opportunities. For all that the momentum seemed to be with Belgium, this was not a composed end to the game from them. They did not simply see it out; they may not quite have been clinging on but it was never comfortable, they never seemed entirely in control.

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Thrilling as they were going forward, this was not at all convincing. In the first half that could perhaps be ascribed to circumstance, but not later on. The biggest problem is that this side, in a progressive 3-4-3, are designed to attack. Even if Witsel is fit enough to start from the knockouts onwards, even with Lukaku’s ability as the ultimate in de luxe target men, physical power matched with pace and tactical and technical ability, even with the class of De Bruyne and Eden Hazard, the back three cannot sit too deep. It has to link to the midfield. With three players in their mid-30s, plus Denayer and Boyata, the lack of pace at the back is a major issue.

The clock has been ticking on this golden generation for a while; with 10 players aged 30 or more in the squad (and De Bruyne to turn 30 before the final) the alarm may soon be going off.