Bruno Guimarães said Arsenal's "unacceptable" performance in their defeat at Brighton was not fit for a Premier League champions' shirt.
Arsenal's 100% start to the new season ended in abrupt fashion as Brighton put Mikel Arteta's side to the sword with a brilliant 3-0 victory.
The Gunners, who arrived at the Amex Stadium with seven wins from their seven matches so far, were well off the pace.
And Guimaraes, after making his first league start for the Gunners since his £75 million ($100m) transfer from Newcastle, said: "We are very disappointed, and we have to learn from our mistakes because we were far away off our level.
"We were too sloppy with the ball, didn't win any duels. And we cannot accept this while wearing this shirt of a champion. We cannot repeat this.
"It's a lesson for us because I think we've been on a good run so far. We'd won every game, but this is a game to forget and we have to improve. It was a bad day at the office for everyone, and we have to say sorry to the fans because it wasn't good enough."
Brighton supporters revelled in their fine win against Arsenal and goaded Arteta's men with chants of "Are you Tottenham in disguise?" and "Champions of England, you're having a laugh."
Manager Arteta will now have three weeks to stew on Arsenal's heaviest defeat since 2023, with his side not back in action until a home match against Leeds on October 10.
However, defender Ezri Konsa has challenged his teammates to use the extended international break to "reflect and learn" on a result which derails any suggestion Arsenal will canter to a second league title in as many seasons.
"We have high standards here and we let them go," Konsa said. "We are very disappointed to lose and to concede three goals is not like us. As a defender, I pride myself on keeping clean sheets.
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"We conceded two goals from outside of the box. They were two very good strikes.
"But there were moments in those two goals where we could have pressed the ball more, and put more pressure on the ball.
"It's the Premier League, when you give quality players outside the box time and they shoot, it can go in.
"But it is important we use this time now to reflect as individuals, we have to move on very quickly and know that when we do come back there is a lot to learn and a lot to fix from this game."