Stoppage time provided a vignette of classic latter-Wenger Arsenal. Hearts were in mouths, and nobody knew whether to celebrate or panic or faint. Having taken a lead, then thrown a lead, and then seized back some pride with a last gasp equaliser, Arsenal’s defence evaporated completely as Chelsea almost won it all over again. “Never a dull moment,” muttered one fan to his mate as they shuffled out the stadium having witnessed another night of strangely confusing excitement against a top opponent.

Arsène Wenger recently said, albeit tongue in cheek, that he would prefer to play with one defender. Somewhere in there is a grain of truth, in that he has always been in thrall to the idea of what to do with the ball. Without it seems more of an afterthought. Make that an after-afterthought.

There was a telling moment midway through the second half of this absorbing contest, just after Jack Wilshere careered off to live the happiness of a big goal. An old chant, not so often heard these days, was reprised: “One nil to the Arsenal!” Duller moments back then, maybe, when a brilliantly drilled defence could be relied upon to tightly link defensive arms and sew up games as if it was an affront to even think about conceding a daft goal. But today’s team is built differently. Flashes of attacking inspiration float about, not anchored by enough ballast.

This one-nil lasted four minutes. Arsenal’s shaky defensive habits are perhaps explainable given they are not a team designed with serious containment in mind. An eminent Arsenal defender of old told a story the other day about a conversation with his former boss about how exposed it can leave a team to allow two full backs to bomb upfield at the same time. In the early days of Wenger if one went, the other stayed. So why abandon that? Wenger’s response was a simple one, which gets to the heart of his philosophy: “We play.”

While the policy remains entrenched Arsenal will strain to be serious contenders. The open moments of this absorbing derby were enjoyable to watch but also startling. A quarter of an hour in the gasps cascaded around the Emirates crowd as Victor Moses floated a long pass forwards and Arsenal’s defence seemed afflicted by some kind of confused dizziness. Everything was a slow blur with the exception of Alvaro Morata, sprinting in between Calum Chambers and Shkodran Mustafi as if he was in a different time zone. The Spaniard somehow squandered the gift of a free shot.

Arsenal have used ten players in their back three this season, including some surprises such as Francis Coquelin and Mohamed Elneny, more commonly found cantering modestly in central midfield. Full-backs and wing-backs have been chucked in too and asked to do a job they are not used to. The overall strategy doesn’t feel particularly organised.

Wenger’s team have recent history in confounding defensive expectation against Chelsea from last May’s FA Cup final, where Per Mertesacker summoned his inner phoenix standing tall amid the flames and Rob Holding relished the opportunity to show Diego Costa that he was in no mood to be cowed.

Arsenal’s Mesut Özil has played with exemplary effort and elan in recent weeks. Photograph: Stuart MacFarlane/Arsenal FC via Getty Images

Here we were. Chambers, Mustafi and Holding this time. A degree of anxiousness filtered through them. Eden Hazard’s nimble dribbles frightened the shorts off Arsenal’s defence, and runners were able to amble through in support. With better finishing Chelsea could have caused plenty of damage.

Arsenal’s way these days asks a lot of their forward players as they can’t rely upon clean sheets. Mesut Özil again relished the responsibility. He has played with exemplary effort and élan in recent weeks, and again picked up the baton to try to show leadership from his posting in the spaces between the midfield and attacking lines.

Özil and Alexis Sánchez connected, as they tend to do. There is promise in this probably short-lived front three, with Alexandre Lacazette causing problems with his runs even if he is working through a frustrating streak the chances not falling, the touch not instinctive. That proved not to be a problem for the revived Wilshere, unerring when the chance fell his way.

But back came Chelsea, with Hazard at the heart of it, earning a penalty when Héctor Bellerín stretched and caught his opponent’s foot, before Marcos Alonso stole in front of Mustafi in a manner that summed up another of those costly little lapses Arsenal are prone to at the back.

Arsenal took solace in their resolve to come back through Bellerín’s sweet strike. But deep down they must know they can only go so far with their current make-up. In home games against the best recently they had a go in spite of defensive unravelling. However much you want to just play, defending should not be some kind of afterthought.