With Kai Havertz having sent Arsenal ahead, Declan Rice was sent off on 49 minutes before Joao Pedro struck Brighton's equaliser.
“We started the game really well,” Arteta said. “We created three or four big chances that we didn’t convert, unfortunately. Then we had a period where we had some problems, especially the giveaway that we gave after regaining the ball, but we scored a really good goal.
“We were on top of the match, we started the second half really well and, in that moment, the referee makes a decision that obviously changes the course of the game for us. And then still like this, the team reacted amazingly, the crowd reacted amazingly, and we should have won the game.
“We had to adapt to the context, we thought it was very inconsistent.
“And he made one decision, by the book he can make, but he totally missed to make the second one, and we should have played 10 against 10.
“Now we have to think about what we have done in the first few games, start to prepare for the next cycle, make sure that everybody that’s on international duty comes back with good health, and ready for the next things.”