VIDEO: Per Mertesacker earns early red card for scything down Diego Costa
Arsenal forced itself into a difficult spot, having to play a man down for the majority of their match against Chelsea after defender Per Mertesacker was sent off in just the 18th minute.
Willian played a fantastic through ball for Costa that cut around Mertesacker, and the towering German was forced to turn and chase, something not exactly his forté.
Mertesacker decided to cut down Costa in his tracks well before the striker reached the penalty area, and referee Mark Clattenburg waited a few moments to make his decision before pulling out the red card and sending Mertesacker to an early shower.
The Blues took advantage and scored the opener just four minutes later as Costa headed home a corner past Petr Cech.