COMPLACENCY often comes before a sudden shock, and there was a bruising reminder that Arsenal do have vulnerabilities, and that defending the league title will be just as hard as winning it last year – if not harder.

It’s not that the Gunners’ manager, Mikel Arteta, ever takes any result for granted, approaching each match with a stubborn intensity, but you got the impression that the away fans filing into the Amex Stadium on Saturday believed this would just be another notch on the gun. Those tense, worrisome matches in late April and May have been replaced with beaming confidence that Arsenal can and will win every match.

Brighton, celebrating their 125th anniversary with cup final-style flags and fireworks, have already shown that reputation means little once you hit the Sussex seaside. They did not give Arsenal an inch in this contest and were thoroughly deserving surprise winners. The high-energy press, which offered Arteta’s men no room to thread passing moves together, set the tone for an almost one-sided match and perhaps offered a blueprint to others of how to frustrate the champions.

Unable to find their groove, Arsenal found themselves behind when Pascal Gross was allowed too much time on the edge of the box to pierce a shot past David Raya, in off the post. There was no plan to sit on this 31st-minute strike, and the Gunners were hounded for the ball at every touch.

Perhaps things could have been different if Martin Odegaard and Bukayo Saka had not made a hash of a very promising breakaway. No sooner had Saka failed to beat Bart Verbruggen from close range than the Seagulls were two up and in proper pecking mode. Again the defence stood off Brighton’s forwards, allowing Charalampos Kostoulas to unleash a shot, again from long range. Even David Raya could not reach it at full stretch.

You might have expected more of a reaction coming out of the break, and there were some promising moments for Arsenal in the second period, but it was a case of a decreasing number of their own supporters staying in the ground to see them. Once a third went in for Brighton – Chema Andres heading in a corner – a fair few decided to try and beat the train queues.

Kai Havertz, on one of his bad days, fluffed his lines in a one-on-one with Verbruggen, pushing a tame shot into the keeper’s grateful arms. But like Havertz, many had an off day. The new centre-back from Aston Villa, Ezri Konsa, did lots of shouting and pointing, and a fair bit of moaning – but not enough with his feet. Bukayo Saka couldn’t beat his man, Bruno Guimaraes took too long and Christos Tzolis struggled all afternoon.

There was much credit due to Brighton, a club that is consistently stronger than the sum of its parts and has a shrewd tactician in manager Fabian Hurzeler. But Arsenal, who never seem quite comfortable with this particular fixture, had a lot of reasons to blame themselves for the final scoreline. If anything, the home side could have had more, and Raya was needed repeatedly to keep this as respectable as it could be.

Arteta said afterwards: “First of all with the performance and then with the result. I think we need to give credit to Brighton as well, the manner that they played today, the way they competed today, and overall I think they deserve to win the game. A lot was down to us, certain standards and basic things that we didn’t do at all. That put us on the back foot right from the beginning of the match.

“We need to feel the pain. That’s not going to take away our perspective and how much we value what the team has done in the first six or seven weeks of the season. Coming back after the World Cup with 18 players involved, with everybody coming at late stages, the way we have competed, the way we have played, the way we have dominated. It’s been really good. But today we need to take this lesson very, very seriously because it’s going to happen.”

Asked if he saw Declan Rice and Bruno Guimaraes as a regular future midfield pairing: “It’s one of them, and it worked extremely well when we played Sunderland away and managed to win the game, and today it didn’t work so well for a lot of things that happened behind them as well to make sure they could play a completely different game. It’s something that will for sure evolve, they’ve only played together a few minutes in these early stages, and that relationship will for sure improve and give us a lot of joy.”