Craig Dawson's double piled more pressure on Arsenal boss Arsene Wenger as the Gunners lost 3-1 at West Bromwich Albion.
Dawson and substitute Hal Robson-Kanu, who scored just 75 seconds after coming on, inflicted a sixth loss on the beleaguered Wenger in his last nine games.
Alexis Sanchez's 18th Premier League goal of the season had cancelled out Dawson's opener but the Gunners remain five points adrift of the top four with their hopes of Champions League qualification in the balance.
They also lost Petr Cech to injury and replacement David Ospina was at fault for Robson-Kanu's goal after colliding with Hector Bellerin.