Arsenal “had no time to prepare players” after Euro 2016 and lacked experience in their 4-3 loss to Liverpool, said manager Arsene Wenger.
The Gunners, who led 1-0, conceded three times in 14 second-half minutes as they started the new Premier League season with a home defeat.
“Some players who came back late were physically short,” added Wenger, who was booed by some fans at the Emirates.
“We paid for that. It’s a situation where you can only be punished.”
Midfielder Aaron Ramsey, who helped Wales reach the semi-finals at Euro 2016, had to be replaced after 61 minutes with a suspected hamstring injury.
“We pushed players like Ramsey and he got injured again,” said Wenger.
“It’s an impossible situation to get the players back from the Euros and have no time to prepare.”
Arsenal’s next game is at defending champions Leicester City next Saturday.
In a pulsating game, Theo Walcott gave Wenger’s side the lead moments after missing a penalty before Liverpool hit back through two goals from Philippe Coutinho and one apiece from Adam Lallana and new signing Sadio Mane.
Liverpool boss Jurgen Klopp celebrated Mane’s goal with his players, before Alex Oxlade-Chamberlain and Calum Chambers scored for Arsenal.
Klopp said his side had “switched off” – and that he was to blame.
“I have a big responsibility for the last half-an-hour,” he added.
“It’s not allowed that a manager celebrates a goal like this. I knew immediately it was a mistake by me.”