Arsenal, making their first appearance in Europe's elite club competition in six years, were 3-0 up at halftime thanks to goals from Bukayo Saka, Leandro Trossard and Gabriel Jesus.
Manchester United goalkeeper Andre Onana took responsibility for his side's 4-3 Champions League defeat by Bayern Munich on Wednesday after his error for the opening goal handed the German giants the initiative.
Jude Bellingham scored deep in added time as Real Madrid wore down a resilient Union Berlin to claim a 1-0 win at the Santiago Bernabeu in their Group C opener on Wednesday, spoiling the Bundesliga side's Champions League debut.
Manchester United coach Erik ten Hag on Tuesday backed his side to come through their early-season struggles ahead of Wednesday's Champions League tie with Bayern Munich.
Newcastle held AC Milan to a goalless draw in Tuesday's opening match of their first Champions League campaign in two decades.
Mikel Arteta has told his Arsenal players to make "beautiful memories" on their return to the Champions League after a six-year absence.
Manchester City begin their defence of the Champions League on Tuesday when the group stage kicks off in the last season in its current format as Europe's elite club competition gets ready for a radical change.
Barcelona coach Xavi Hernandez said Monday his time in the team as a player helped set a high bar which the club are now struggling to reach in the Champions League.
An obsession with winning the Champions League would only hurt Paris St Germain, manager Luis Enrique said on the eve of the Ligue 1 side's group-stage clash with Borussia Dortmund.