Brazilian teenager Marquinhos scored on his Arsenal debut as the Gunners started their Europa League campaign with a 2-1 win over FC Zurich in Switzerland on Thursday, while Jose Mourinho's Roma slumped to a surprise defeat.
Nineteen-year-old Marquinhos, signed from Sao Paulo in June, finished first-time from a low Eddie Nketiah cross in the 16th minute to put Arsenal ahead in the city of St Gallen.
Zurich, who were Swiss champions last season for the first time since 2009, equalised just before half-time when Mirlind Kryeziu converted a penalty after a foul in the box by Nketiah.
However, Nketiah made up for that by scoring the winning goal just after the hour mark, heading in a hanging cross from Marquinhos.
The game took place as news of the death of Britain's Queen Elizabeth II came through, and the two sets of players held a minute's silence before the start of the second half.
The win made it the perfect start in Group A for Arsenal, after coach Mikel Arteta made eight changes to his line-up following the 3-1 weekend defeat at Manchester United in the Premier League.
Gabriel Jesus, Martin Odegaard and Aaron Ramsdale were among the players rested while Marquinhos made his first appearance, as did American goalkeeper Matt Turner, signed in the close season from New England Revolution.
Fabio Vieira also made a first start since joining from Porto.
Group rivals PSV Eindhoven and Bodo/Glimt drew 1-1 in the Netherlands, with Cody Gakpo equalising for Ruud van Nistelrooy's side after Albert Gronbaek had given the Norwegian champions the lead.
Arsenal welcome PSV to the Emirates Stadium next Thursday.
Roma are looking to claim a second succesive European trophy after winning the inaugural Europa Conference League last season, but they went down to a surprise 2-1 loss away to Luodogorets Razgrad in Bulgaria in Group C.
Uzbek forward Eldor Shomurodov looked to have secured a draw for Roma when he equalised with four minutes to go, cancelling out Brazilian midfielder Cauly's opener.
But the Bulgarian champions grabbed all three points in the 88th minute through Nonato, another member of their sizeable Brazilian contingent.
Meanwhile Fenerbahce beat Dynamo Kyiv 2-1 in Istanbul in Group B with Michy Batshuayi coming off the bench to score an injury-time winner on his debut following his recent move from Chelsea.
The result is a measure of revenge for the Turkish side, who lost to the Ukrainians in the Champions League qualifying rounds.
Dynamo were then beaten by Benfica in the Champions League play-offs, and so the clubs were reunited in the same Europa League group.
Manchester United were among the clubs in action later as they hosted Real Sociedad at Old Trafford in Group E.