The striker pounced on a Sebastian Coates error and slotted a shot through the keeper's legs after 77 minutes.

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Earlier in the second half, Welbeck had a headed goal ruled out after he was judged to have fouled the defender in the build-up.

Arsenal have now extended their winning run to 11 games in all competitions.

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This was Arsenal's first away win in Portugal after previously failing to come away victorious in six European visits there (D3, L3).