Arteta's Gunners were pegged back by Brighton as Nicolas Pepe's 68th-minute opener was cancelled out by Lewis Dunk in the 75th minute before Neal Maupay wrapped up the win in the dying minute.
Mikel Arteta was disappointed with how Arsenal fell to an "unacceptable" 2-1 Premier League defeat to Brighton and Hove Albion on Saturday.
The Gunners scored the first goal of the game at the Amex Stadium through a brilliant strike from Nicolas Pepe but they failed to increase the lead and were pegged back by goals from Lewis Dunk and Neal Maupay.
This was Arsenal's second defeat in a week following their 3-0 loss to Manchester City on Wednesday and Arteta revealed his anger towards the result.
"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."
He added: "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."
On Leno, who got injured, he said: "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."