London (PA Media/dpa)
Altay Bayindir's blunder handed Arsenal victory and undid a positive Manchester United performance in Sunday's pulsating Premier League encounter.
Old Trafford rocked as Mikel Arteta's runners-up headed to the rebuilding Red Devils in the most eye-catching fixture of the opening weekend.
United looked far better than the side that finished 15th last season - their worst top-flight campaign since 1974 - yet Ruben Amorim's side fell to a 1-0 defeat as Riccardo Calafiori capitalized on Bayindir's early flap.
The back-up goalkeeper was outmuscled by William Saliba as he attempted to deal with a Declan Rice corner, tipping it on for the defender to nod into an empty net and silence Old Trafford after a bright start.
Bayindir's error underlined concerns over the goalkeeping position at Old Trafford, where often unconvincing number one Andre Onana was absent as he works his way back from a hamstring injury.
United are a work in progress but new signings Matheus Cunha and Bryan Mbeumo looked sharp on their debuts, with Patrick Dorgu hitting a post with a lasered first-half effort.
Arsenal keeper David Raya produced a string of important stops to keep the hosts at bay, with his man of the match of the display helping Arteta's side to victory.