If there ever was a sign that Sydney FC cannot take the A-League title for granted, it is their 2-1 loss to a Newcastle side playing almost 80 minutes with 10 men.
Saturday's one-versus-two grand-final rehearsal lived up to the hype, leading to the Sky Blues' second loss this season.
Newcastle's Roy O'Donovan was sent off in the 14th minute for punching Jordy Buijs in the face after an altercation while contesting the ball.
A four-minute VAR review eventually revealed the Irish striker had swung his closed fist across the Dutchman's face while holding onto his shoulder, leading to referee Chris Beath red-carding him.
But Buijs later turned villain after conceding the penalty for the Jets' opening goal.
A boisterous 18,156-strong crowd - the A-League's biggest in almost five years at McDonald Jones Stadium - watched Ernie Merrick's entertainers prove they had the mettle to surge from wooden-spooners to champions.
Merrick praised an exceptional defensive performance.
"I'd say the boys are pretty confident they can match it with anyone now," Merrick said.
Arnold admitted his side lacked its customary crispness and execution.
"That was the worst thing that happened to us, Roy getting sent off and them going down to 10 men," Arnold said.
"Then it becomes a bit of an attitude problem."
Based on Sydney's ruthlessness, the numerical disadvantage should have killed off the contest after O'Donovan's dismissal.
But the Newcastle defence - boosted by the inclusion of Nigel Boogaard, who re-injured his knee late - settled in and Daniel Georgievski contained Adrian Mierzejewski.
The Sky Blues looked to capitalise and came close on 38 minutes when Milos Ninkovic brushed the side netting off Mierzejewski's misguided cross.
Soon after Nikolai Topor-Stanley drew a relatively soft penalty from Buijs before Dimi Petratos converted with ease.
The Sky Blues returned fire soon after the break, Bobo rising to head home a wonderful Buijs free-kick for the Brazilian golden boot leader's 22nd goal this season.
The ledger was level for just five minutes before Nabbout barrelled in another stunner from distance - an angled 30-metre effort that sailed past a slow-reacting Andrew Redmayne into the far corner.
As the Sky Blues started to turn the screws, Georgievski stuck out his chest to clear Mierzejewski's deflected shot off the goalline and save the game.
Australian Associated Press