This was another result very much in keeping with a Premier League campaign in which the big beasts, with the exception of Chelsea, continue to toil as Swansea fought back from a goal down to stun the Gunners.
Having trailed to an Alexis Sanchez opener, two second-half substitutions from Garry Monk were rewarded as Modou Barrow won the free-kick which led to Gylfi Sigurdsson’s equaliser before Bafetimbi Gomis headed home a dramatic winner.
It was enough for them to leapfrog Arsenal into fifth — capping a miserable week for Wenger after his team blew a 3-0 lead to draw in the Champions League in midweek.
They are yet to beat a side outside the bottom seven in the Premier League.
“I worry that we concede the goals, yes,” Wenger said. “At the moment we have different worries than that [the gap between Arsenal and Chelsea]. We just lost a game that we feel we should not have lost. You have to win the games you can win and today was one. If you want to challenge for the Premier League you have to be a bit more realistic.
“It is difficult to explain how we lost the lead. We were in control but they did not give up and we lost some decisive challenges in the middle of the park and we paid for that. That is where we lost the battle.”
This contest was supposed to pit two of the Premier League’s artists together, yet the first half failed to live up to that billing.
Swansea, initially, were the more purposeful and it required Calum Chambers’ goal-line heroics to deal with Sigurdsson’s viciously-whipped free-kick which was threatening to creep inside the far post.
The Arsenal defender could, however, count himself fortunate in the 22nd minute when a shove into the back of Wilfried Bony inside the penalty box which floored the Ivory Coast international went unpunished.
For all the plaudits Chambers has received since his arrival from Southampton, there remain chinks in his defensive armour. And his youthful naivete was exposed soon afterwards when an untimely slip near the halfway line gifted Montero the opportunity to counter.
Rapidly the play was switched from left to right, culminating in Bony slipping in the overlapping Marvin Emnes, yet his shot was too close to Szczesny.
A hangover from that sloppy surrender to Anderlecht? On the early evidence, yes. Yet there was at least a flourish to finish the first half from the visitors.
Their first genuine opening arrived in the 45th minute as former Gunners goalkeeper Lukasz Fabianski foiled Danny Welbeck at the near post following a liquid interchange of Gunners passes. That was more like it. Nor did it end there as Aaron Ramsey, with consummate timing, volleyed inches past the post.
Wenger resisted the temptation to make changes and, after Alex Oxlade-Chamberlain had spurned a golden chance when sent clean through within seconds of the restart, the Arsenal manager’s patience was finally rewarded.
It was a counter-attack of clinical precision. Oxlade-Chamberlain and Santi Cazorla played their part before Welbeck, on the right side of the area, cleverly cut inside with a drop of the shoulder, buying him the time to pick out Sanchez inside the area.
He, of course, made no mistake, ramming home to make it six in his last four games.
Then came that feeling of déjà vu.
Any initial relief that Kieran Gibbs had taken one for the team by cynically pole-axing Barrow as he charged goalwards proved short-lived as Sigurdsson whipped a wonderful 75th-minute free-kick over the wall and inside the left upright.
Monk was unmoved, instead focused on his next move. On came Gomis for Bony and, within two minutes, it had proved a masterstroke.
Chambers, once again, had no answer to Montero’s searing speed and the winger picked out Gomis, who headed home.
“He is very selfless,” Monk said. “He has waited for his chance and got his reward. You could see in the emotion he had how much it meant.”
Wenger called for the cavalry. Theo Walcott and Jack Wilshere were introduced. It was, however too little, too late.
SWANSEA: Fabianski, Rangel, Bartley, Williams, Taylor, Ki, Carroll (Britton 87), Emnes (Barrow 67), Sigurdsson, Montero, Bony (Gomis 76).
ARSENAL: Szczesny, Chambers (Sanogo 90), Mertesacker, Monreal, Gibbs, Ramsey (Walcott 79), Flamini (Wilshere 80), Oxlade-Chamberlain, Sanchez, Cazorla, Welbeck.
Referee: Phil Dowd.