The left-back was initially yellow carded by Craig Pawson but the referee changed his mind and upped his punishment to a red after watching the replay, judging the Arsenal star to have denied a goalscoring opportunity.
JARROD BOWEN put another dent in Arsenal's title hopes as the Gunners suffered their first home Prem defeat of the season.
West Ham made it back-to-back wins at the Emirates and were helped by Myles Lewis-Skelly's red card with 20 minutes to go.
Even with 11 men the Gunners struggled to break down their opponents, with their lack of striker proving costly once again.
Arsenal are now eight points off league leaders Liverpool having played the same number of matches.
