Manager Mikel Arteta fielded an almost completely new starting eleven compared to the meeting against Liverpool in the league last Thursday.
Only center-back Gabriel remained in the team.
That likely explains the confused defensive play that Portsmouth immediately exploited. Colby Bishop scored 1–0 in the third minute.
But conceding corners to Arsenal is usually costly.
Arsenal had two in the first 25 minutes - and both paid dividends. The first came through an own goal and the second via a header by Gabriel Martinelli.
The set-piece statistics could have been improved, but Noni Madueke missed a penalty.
Kai Havertz made a comeback in the second half. The match was the German's first for Arsenal since he was injured in August. Arsenal were leading 3-1 after a goal from Martinelli. The Brazilian completed his hat-trick when he headed in to make it 4-1.
For Arsenal, 16-year-old defender Marli Salmon made his debut as a substitute. Swedish national team striker Viktor Gyökeres sat on the bench for the entire match.
Fellow Swede Yasin Ayari was not in Brighton's starting line-up for the trip to Manchester United. The team still did well, winning 2-1.
Brajan Gruda's opening goal was followed by a magnificent goal by veteran Danny Welbeck, who once played for United. Ayari played the final quarter.
United have not yet announced who will take over permanently as manager after sacking Ruben Amorim.
The fourth round of the FA Cup will be drawn shortly before Monday night's meeting between Liverpool and Barnsley.
