The home side fell behind after Shane Long stole a march on Shkodran Mustafi to prod the ball past Petr Cech, before Pierre-Emerick Aubameyang restored parity with a neat finish after a delicate touch from Welbeck.
Arsenal and Egyptian national team midfielder Mohamed Elneny became the first Egyptian to be sent off this season in any of the European leagues in the Sunday game against Southampton.
The result was an attack with less playmaking ability but a lot more pace, and they always looked a lot more unsafe on quick counters than when trying to orchestrate the normal slow build-up moves.
The former Manchester United man then gave his team the advantage with a deflected strike after cutting in from the left, but the Gunners were again guilty of slack defending as Charlie Austin levelled the scores with 17 minutes left on the clock.
The Gunners are unbeaten in their last 22 home league games against Southampton (W16 D6), since a 0-1 loss in November 1987. They've not lost three consecutively by three or more goals in league competition since August 1930 in the second tier.
Yet they will return to the south coast with nothing to show for their efforts, Welbeck heading in the game's decisive fifth goal before tensions boiled over during added time.
- Danny Welbeck both scored and assisted in a Premier League match for the first time since September (v Bournemouth), which was also the last time he netted a league goal. His winning goal was just reward for his perseverance after a shocking open goal miss from close range minutes before.
Alexandre Lacazette admits to having sought advice from Arsenal legend Thierry Henry during a testing debut campaign at Emirates Stadium. It wasn't for a want of trying, though.
Defeat for Southampton leaves them in the relegation zone and three points from safety after they lost their early lead here.