Mikel Arteta said Arsenal's 3-0 defeat by Brighton was a reminder of "how tough every game" in the Premier League is as the defending champions were beaten for the first time this season.
Brighton dominated from the off at the Amex Stadium and punched holes in Arsenal's normally resolute defence thanks to long-range strikes from Pascal Gross and Charalampos Kostoulas before halftime.
Chema Andres rose highest to head in a third from a corner early in the second period as Arsenal lost by more than two goals for the first time in over three years.
"Extremely disappointed. First of all I think we have to congratulate Brighton for the victory, I think they deserve it," said Arteta.
"I think they were the better team. From our side there were certain basics that you need to accomplish in football, especially in the Premier League, that we were very far from."
The Gunners have been tipped to cruise to the title after ending their 22-year wait to be crowned champions of England last season.
Arsenal started the campaign in ominous fashion, winning all seven games before Saturday in all competitions.
But Arteta said defeat served a timely reminder ahead of a three-week international break of how tough the season will be.
"It's a big lesson for us. Is that a reminder to some of the players how tough every game is," added the Spaniard.
"That's not going to take away what the team has done and the boys have done in the last six or seven weeks since coming back from the World Cup.
"With 18 players coming back, what we've done and the way we've competed and the amount of games we've won has been remarkable.
"But this is a big lesson for all of us to understand what it takes to win at this level."