Bellerin’s mistakes at the other end remained a problem, with Sokratis Papastathopoulos forced into a desperate block from Felipe Anderson before the Spain full-back played Robert Snodgrass onside to shoot too close to Petr Cech.
Cech stood firm to thwart Arnautovic in the 55th minute and apparent knee injury he sustained during that attack proved pivotal.
Diop allowed Alexandre Lacazette too much room to turn and shoot, with ex-Arsenal goalkeeper Lukasz Fabianski beating the shot away, and the 21-year-old centre-back was the architect of his own misery 20 minutes from time.
A miscued clearance spiralled up in the air and Aaron Ramsey nodded down for Lacazette to drive into the goalmouth, with Diop only able to divert into his own net.
Felipe Anderson continued to probe, flashing agonisingly across the face of goal in injury time, meaning Arsenal nerves only settled when Welbeck swivelled to dispatch Bellerin’s pass.
Some sharper finishing from the Austria forward might have seen West Ham take all the points back across London but Arnautovic was very impressive as their focal point, revealling in Arsenal’s defensive uncertainty. His goal was well taken and the 29-year-old – handed the captain’s armband by Pellegrini – performed with a level of maturity that has not always been evident during a career that has thrilled and frustrated in equal measure.
A clash of styles awaits as Arsenal travel to Cardiff City next weekend, with West Ham hosting Wolves on the back of a another all-London affair against AFC Wimbledon in the EFL Cup.
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