Arsenal were kept at bay for 42 minutes by Dundalk before ultimately easing to a comfortable Europa League victory.
Rangers made it eight wins in a row across all competitions thanks to Alfredo Morelos' intervention against Lech Poznan.
Dele Alli and Gareth Bale failed to make the most of Tottenham starts as they were substituted in a dismal defeat at Royal Antwerp.
Brahim Diaz, Rafael Leao and Diogo Dalot were on target as Milan made it two wins from two in this season's Europa League.
Jose Mourinho made eight changes to his starting line-up for the first game of the Europa League group stages but oversaw a comfortable win.
Atletico Madrid won the Europa League with a 3-0 victory over Marseille, as Antoine Griezmann played a starring role.