Mikel Arteta slammed Arsenal for an "unacceptable" 2-1 Premier League defeat to Brighton and Hove Albion on Saturday.
The Gunners went ahead at Amex Stadium through a sublime Nicolas Pepe strike but were stunned by goals from Lewis Dunk and Neal Maupay.
Dunk scrambled the ball over the line after a quick Brighton corner before a 95th-minute winner from Maupay, who was involved in an earlier collision that saw Gunners goalkeeper Bernd Leno taken off on a stretcher.
It was Arsenal's second defeat in a week following their 3-0 loss to Manchester City on Wednesday and Arteta was far from happy with the way they failed to hold onto their earlier advantage.
"I am very frustrated," he told BBC Sport. "It is unacceptable the way we lost the game.
"We like competing and I know we have a young squad but still, we threw the game away. We had to put it to bed earlier."
Leno appeared very unhappy with Maupay as he was taken off in the first half and there was a scuffle on the pitch after the final whistle in which the Brighton striker was involved.
Arteta was not inclined to blame Maupay for his goalkeeper's injury but admitted the problem looks serious.
"I think it was very unfortunate," he said. "I don't think he wanted to hurt him. It is not looking good. We have to fight and be positive; tomorrow is a different day.
"I think the players that came on today did really well, we did more than enough to win the game comfortably, but we leave with zero points again.
"I think this was a very different one to the one against Manchester City. When you give cheap goals like we did in this one you will suffer."
Emiliano Martinez, who replaced Leno in goal, also criticised his team-mates for "switching off" at the worst moments.
"We switched off and when you switch off in the Premier League you get punished," he said.
He added on Leno: "I spoke to him at half time and he felt he twisted his knee. As a goalkeeper, you do not want him to get injured. It is a big loss to the team.
"I don't think he [Maupay] tried to injure him. He went for the ball like any striker would do and this is football. It can happen.
"You train hard and have to wait for your chance. I am more than ready."