After the knockout blow came the Saka punch.
It won't bring back the FA Cup they surrendered so fecklessly at the weekend, but rising star Bukayo Saka led an inspired display of counter-attacking football to restore a measure of pride in Arsenal's unfathomable season.
Barely 72 hours after their feeble defence of the Cup had expired at St Mary's, the Gunners could have been forgiven for shrinking to the occasion like crash victims returning to the scene where they wrote off their Lamborghinis.
And in an evil, swirling drizzle – the sort of murky night better suited to the Hound of the Baskervilles than football – this was a defining test of their appetite for getting stuck in.
How the mighty are fallen. After 22 years of painting masterpieces under Arsene Wenger, now they are judged on whether they fancy creosoting the garden fence when it's wet.
But they were good value for three points. Saka was outstanding, Rob Holding and David Luiz gave Saints warrior Danny Ings less change than a fruit machine and, finally, the Premier League table no longer looks like a threatening letter from Arsenal's bank manager.
This was all the more impressive because they fell behind inside three minutes as Stuart Armstrong met James Ward-Prowse's corner with an exquisite half-volley without breaking stride.
Unlike their lukewarm retreat in the Cup, Arsenal's response was tenacious this time.
Pressing high and forcing Jack Stephens into a mistake, they were level within five minutes as £72 million flop Nicolas Pepe supplied a cool finish to Granit Xhaka's pass.
Mikel Arteta's Gunners have been starved of title glory for a while now - but things could be changing as a new-look Arsenal side plot a course to success.
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Saka's twinkling feet and remarkable versatility are precious assets for fallen giants trying to find their way out of the mid-table jungle, and six minutes before the break their value soared again.
Surging clear from Lacazette's instinctive pass, the England winger skipped round Alex McCarthy to put the Gunners in front.
Saints, without both first-choice full-backs, went through the motions of a fightback, but for once they lacked conviction and purpose.
And 17 minutes from time, they were broken beyond repair by a move of breathtaking simplicity – with Saka yet again at the heart of it.
Cedric Soares picked him out with a diagonal pass of raking quality, Saka's low centre was a perfection of geometry and Lacazette did the rest at the far post.
Suddenly, a season which appeared to be lurching towards unthinkable territory before Christmas doesn't seem quite as forbidding.
Gunners boss Mikel Arteta is up to eighth, within five points of the Champions League places.
And he can thank his lucky stars his boss is not a trigger-happy oligarch who found £250 million down the back of the sofa last summer and expects an instant return on his investment.