Lookman bagged a hat-trick as Atalanta won its first-ever European trophy last season.
Brazilian striker, Gabriel Martinelli, opened the scoring for the Gunners with a brilliant left-foot strike 28 minutes into the game.
English Premier League (EPL) leaders Arsenal were held to a 1-1 draw by Southampton on Sunday.
Arsenal secured their spot in the Europa League knockout stage with two games to spare after beating PSV Eindhoven 1-0 on Thursday.
Ethan Nwaneri became the youngest ever player to feature in a Premier League match at 15years and 181 days.
With Saturday’s win, the Gunners now have six points in two games, maintaining a 100 per cent start to the season.
The development now puts his participation in Sunday’s FIFA World Cup qualifier against Italy is unclear.
Today’s defeat comes after the gunners were trashed in their first and second game by newly promoted Brentford and Chelsea.
