After Mikel Arteta said Arsenal's defending in Wednesday's defeat to Real Betis was "below the standard", the Gunners found themselves in trouble again as they were 2-0 down at the break thanks to goals from Samuele Inacio and 18-year-old wonderkid Konstantinos Karetsas.
Arsenal had plenty of chances throughout - with the wasteful Viktor Gyokeres hitting the bar in the first half - and while Ethan Nwaneri pulled one back after good work from Christos Tzolis, a calamitous corner allowed Joane Gadou to make it 3-1.
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Tzolis then capped off a promising performance by winning a penalty which Gyokeres converted. But with seven days to go until their Community Shield game with Man City in Cardiff, Arteta has some holes to fix.
Arsenal welcomed new signing Bruno Guimaraes onto the Emirates pitch before kick-off - he is needed sooner rather than later.
The key moments from the Emirates Stadium...
7: GOAL! Inacio gives Dortmund the lead after being played through in the box by Lukas Nmecha.
15: NO PENALTY! Odegaard breaks through on goal and is brought down. Referee Chris Kavanagh points to the spot then changes his mind by giving a free-kick on the edge of the box, which Gyokeres fires over.
26: OFF THE BAR! Gyokeres strikes the bar after Odegaard's pass freed Dowman on the right wing.
29: GOAL! Karetsas doubles Dortmund's lead after Riccardo Calafiori gave the ball away on the halfway line, which sparked a solo run.
43: TWO CHANCES! Tzolis produces two glorious free-kicks in the space of four minutes, but Gabriel nods the first one over and Havertz heads straight at the goalkeeper.
55: GOAL! Nwaneri sweeps home to pull one back after good work from Tzolis, who provided the assist down the left.
59: GOAL! Comical Arsenal defending at a corner sees Gadou score a third for Dortmund, after Illan Meslier saved when Calafiori could not clear his lines.
69: GOAL! Gyokeres pulls one back from the penalty spot after livewire Tzolis is fouled by Jobe Bellingham.
90+4: GOAL DISALLOWED! Martinelli thinks he has equalised but is caught offside from a short corner.
Arsenal's pre-season so far...
July 25: Arsenal 3-0 MK Dons - Arsenal training ground
August 1: Girona 1-4 Arsenal - Girona
August 5: Arsenal 1-3 Real Betis - Dublin
August 9: Arsenal 2-3 Borussia Dortmund
August 12: Arsenal vs Como - Emirates Stadium, 7.30pm
August 16: Community Shield - Arsenal vs Man City, Cardiff, 3pm
