One good defensive clearance, and the Foxes would have come away with an invaluable point. Instead, Danny Welbeck's extraordinarily dramatic late header might prove to be absolutely crucial in the title race.
The interesting thing about the goal was the fact it was a header, with Welbeck converting a dinked Mesut Ozil free-kick.
This area of the game was where Arsenal threatened throughout, which is surprising: the Gunners are a technical side that generally focus on intricate passing football, and Leicester are a more old-fashioned, rugged side whose centre-backs are, on paper, poor on the ground but dominant in the air.
Instead, Arsenal chucked the ball into the box frequently, and Leicester struggled to cope.
It wasn't simply Welbeck's header that showed this weakness. Olivier Giroud was a constant threat in the air, and while the Frenchman's headed efforts at goal were generally wayward, his lovely cushioned header for Theo Walcott got Arsenal back in the game.
It was classic Giroud, who is much better with his back to goal playing one-touch lay-offs than when leading the goalscoring himself.
The previous goal Leicester had conceded, in the 3-1 win at Manchester City last weekend, was also a header from a cross – Sergio Aguero converting Bersant Celina's right-wing delivery for City's consolation goal. That strike was, in itself, fairly irrelevant – but it's nevertheless interesting that crosses have led to Leicester's last three concessions, scored by sides who don't particularly like crossing the ball.
This is, in part, because Leicester defend so narrow. Their full-backs tuck inside and protect the centre-backs, covering the width of the penalty area but leaving the flanks bare for opponents to exploit. This makes a lot of sense against a side like Manchester City or Arsenal – it's difficult to remember David Silva or Ozil sliding many through-balls between defenders – but means Leicester must defend a succession of crosses.
Against Man City, Leicester were fortunate that Aleksandar Kolarov's delivery was atrocious. Arsenal, meanwhile, weren't always particularly ambitious with their crossing – lots of looped balls in the vague direction of Giroud – but Leicester were rarely first to the high balls.
Of the 10 aerial challenges inside their own penalty box, Leicester's centre-backs only won two.
Theoretically, the substitution of Riyad Mahrez for Marcin Wasilewski, a replacement right-back following Danny Simpson's red card, should have made Leicester stronger in the air.
Instead, he didn't actually challenge for any high balls, and ultimately conceded the free-kick which resulted in Welbeck's winner. Arsenal's players were queueing up to turn the ball in – if Welbeck wasn't there, Calum Chambers might have nodded it in.
It's also worth remembering that Leicester aren't a particularly big side: of their regular XI (and it's very regular – Claudio Ranieri has named the same XI six times in a row, the first time any Premier League manager has done so this season), only the centre-backs are 6ft or above.
Many sides have full-backs who can defend the back post effectively, a central midfielder who can act as an extra centre-back when the ball is crossed from wide, or a tall centre-forward who can protect the near post zone effectively at set-pieces.
Leicester don't have that – they depend on their centre-backs and goalkeeper to defend aerial balls. Wes Morgan and Robert Huth would probably have been hailed for their heroics had Arsenal not managed their late equaliser but, while they did make some tremendous blocks, they ultimately didn't do the one thing you’d expect: head the ball clear.
We've all been searching for Leicester's weakness over their incredible season so far – maybe they need to work harder to shut down players out wide.
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