Arsenal have lost 2-1 to Brighton after Pepe's second half strike was oveturned by two late goals from the Seagulls.
Mikel Arteta's side made it two defeats in two games since the Premier League returned after a frustrating afternoon at the Amex.
Lewsi Dunk equalised seven minutes after Pepe's excellent 68th minute left-footed curler before Neal Maupay sealed the victory over the Gunners to land the home side's first ever double over the north Londoners.
However, Maupay was viewed as a villain after Arsenal keeper Bernd Leno was stretchered with what looked like a serious knee injury in the 35th minute - following a reckless challenge from Brighton's No18.
Leno received lengthy treatment on the pitch after his knee appeared to give way after he landed awkwardly.
A distressed Leno then sat up as he was being carried off and pointed at Maupay before Emiliano Martinez replaced him for the rest of the match.
The result capped a miserable week for Arsenal, marking their second defeat in two games since the Premier League's restart after losing against Manchester City on Wednesday evening, when Granit Xhaka and Pablo Mari were injured early on.
With Leno out and Brazilian defender David Luiz suspended for two games after being sent off at City, Arteta is facing a selection crisis ahead of next weekend's FA Cup sixth round clash against Sheffield Unitd at Bramall Lane.
The reuslt was Brighton's first win of the year which sees them move five points above bottom three - while Arsenal's bid for European football could be derailed as the club face their worst end to a season for 25 years.