Southampton's hard start continued with Harry Maguire's stoppage-time victor seeing Leicester City earn a 2-1 comeback win, a victory that will have brought much pleasure to visiting manager Claude Puel on his return to his former club.
When Felipe Anderson took possession in midfield, he was able to advanced unchecked towards the Arsenal penalty area, where he fed a pass into Arnautovic.
Their tactic of playing out from the back with Shkodran Mustafi and Sokratis Papastathopoulos split either side of Cech was too easy for West Ham to disrupt, particularly with neither centre-back looking overly comfortable or mobile enough on the ball.
Aaron Ramsey nodded in the rebound but was flagged offside, before West Ham broke and Arnautovic arrowed an effort into the side-netting.
Emery's men responded well to their latest setback, levelling five minutes later as Monreal was on hand at the back-post to turn home Hector Bellerin's deflected cross.
Here are the match highlights (will open in a separate tab).
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We lost ground, we were not able to press, we were not able to defend, we were really in trouble. Sarri admitted his side suffered a "horrible" 15 minutes which could have cost them dear.
Arsenal, finally, saw the winning side of things after they defeated West Ham, last night with the scoreline reading 3-1.
Pierre-Emerick Aubameyang missed a host of key chances against Chelsea as the Gunners looked to find a way back into the match last weekend.
West Ham have made a similar stuttering start to the league campaign after defeats to Liverpool & Bournemouth.
While the east Londoners are still looking to get off the mark this season, Pellegrini is seeing signs of life at the club.
Arsenal's French striker Alexandre Lacazette celebrates their second goal after his cross was turned in for an own goal during the English Premier League football match between Arsenal and West Ham United at the Emirates Stadium in London on August 25, 2018. However the Hammers were undone with two goals in six minutes, including a fine individual strike from Calum Wilson. However, if someone knows how to get the better of Arsenal it would be Wilshere having had ten years with the club.
Emery, after the win, said his players "need to improve".