Arsenal face a daunting task after being drawn alongside last season's runners-up Borussia Dortmund, Rafael Benitez's Napoli and Marseille in the Champions League group stages.
The Gunners, who qualified after completing a comprehensive play-off win over Fenerbahce this week, were placed in the tough-looking Group F at UEFA's draw in Monaco.
Celtic, who famously beat Barcelona last season, will face the Catalan giants again along with two other former winners in Group H in AC Milan and Ajax.
Manchester City will play reigning champions Bayern Munich - who they faced two years ago - CSKA Moscow and Viktoria Plzen in Group D.
Chelsea face two of the sides they conquered en route to last year's Europa League success in Basle and Steaua Bucharest, as well as Schalke, in Group E.
David Moyes' Champions League bow with Manchester United will come against Shakhtar Donetsk, Bayer Leverkusen and Real Sociedad in Group A.