Just when it seemed Jose Mourinho would again prevail in his long-standing feud with Arsene Wenger thanks to a second-half Juan Mata goal, substitute Giroud popped up to head home Alex Oxlade-Chamberlain's cross in the rarest of Arsenal attacks.
Bellerin out for a month, Sanchez in doubt for Man United game
Injury-hit United had largely dominated proceedings and looked set to bring relief to Mourinho after an inconsistent start to the season when Mata struck in the 68th minute.
But in a game that never reached the peaks of the fixture that was once the league's defining contest, there was little consolation for Mourinho in improving his unbeaten league record against Wenger to 12 as Arsenal stretched their unbeaten run in all competitions to 17.
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