Three things we learned from Arsenal loss as Ethan Nwaneri passes audition amid transfer uncertainty
It all felt very low-key for Arsenal at the Emirates Stadium.
Mikel Arteta's side were back on home soil for the first time since beating Burnley in May, a result that was followed 24 hours later by them being crowned Premier League champions.
It was tough to read too much into it. Arsenal are still without their English and Spanish players who were at the World Cup.
However, with the Community Shield only a week away and the Premier League opener against Coventry five days beyond that, Arteta will know his side have to step up significantly.
Arsenal cut open without stars
The concerning signs for Arsenal came with obvious caveats.
Dortmund found it far too easy to cut Arsenal open, charging through midfield and into the final third, and it came as no real surprise.
Arsenal can afford to have Riccardo Calafiori roaming around the pitch and taking risks on the ball when they have Declan Rice and William Saliba in the side.
The pair are supreme physically and can eat up the ground to cover defensively if Arsenal do find themselves exposed.
There was no such protection here. Calafiori lost the ball on the half hour mark and the Gunners never recovered, as Karetsas strolled into space and found the top corner.
That came after Dortmund had cut open Arsenal for their opener, when Samuele Inacio slotted into the bottom corner.
Myles Lewis-Skelly was the deepest midfielder but holding down that space off the ball is not his strength.
Arteta did not particularly need one, but this was a reminder for him of just how much he needs his physical powerhouses.
Tzolis off to flyer for Gunners
His summer arrival came with little fanfare, particularly amid the unsuccessful pursuit of Vinicius Junior, but Tzolis has made a strong start to his Arsenal career.
He had a quiet start to the match in the early stages but still worked hard, tracking his man to concede a corner at the back post.
Tzolis' set-piece delivery will excite Nicolas Jover. He clipped in a couple of brilliant crosses, one of which Gabriel should have headed in.
In the second half, Tzolis stepped up from open play too. He beat his man well on the outside and picked out Nwaneri for Arsenal's opener, and then won a penalty when drifting over to the right wing.
It's still early days, but Tzolis has started life at Arsenal playing with real pace and direct running. Keep up the goal contributions to go with that and he will prove to be a very astute bit of business.
Nwaneri takes opportunity amid uncertainty
Every match feels like an audition for Nwaneri this summer.
The 19-year-old's future at the club remains uncertain after a frustrating loan spell at Marseille last season.
In a transfer window where Arsenal have to be as concerned with their selling as the spending, a significant offer of 'pure profit' for Nwaneri could be tempting.
The teenager certainly took his chance to impress here with a timely performance.
It was noticeable how hard Nwaneri worked. He had his commitment questioned at Marseille but he ran tirelessly here.
He slid in to dispossess Bellingham when tracking back and won it back more than once on the edge of his own box.
Nwaneri got his reward when sweeping in confidently after being picked out by Tzolis. That goal threat is key in a side looking to add more unpredictability in attack.