They suffered their largest ever defeat at the hands of London neighbours Chelsea in the day's opening game in the Premier League, losing 6-0 at Stamford Bridge.
Jose Mourinho's men may also have delivered a killer blow to the Gunners' title ambitions.
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The goals came from Samuel Eto'o & Andre Schurrle, two from Oscar, Mohamed Salah and a penalty from Eden Hazard.
That came after a Hazard shot which was saved by Alex Oxlade-Chamberlain.
However, referee Andre Marriner sent off Kieran Gibbs instead – despite protests from players on both sides that he'd got the wrong man.
