Arsenal manager Arsene Wenger says both conceded goals against Southampton could have been avoided on Tuesday night.
Alexis Sanchez's top-class free-kick gave the Gunners the lead but Ronald Koeman's men fought hard to first restore parity and then take the lead through a superb Nathaniel Clyne strike.
"It's disappointing because we had a good start and we made two unnecessary errors on the two goals - they were big ones," Wenger said in his post-match press conference.
"We lacked a bit of experience at the back. I agree with you but overall they did well.
"We gave a lot and it's frustrating as we were not able to score in the second half.
"When you look at the amount of possession we had you have to give credit to Southampton. They played well and were very organised.
"They abused the fouls in the middle of the park but they are a good side and they played well tonight. Overall it's frustrating because we gave a lot."
Regular defenders Laurent Koscielny, Kieran Gibbs and Per Mertesacker didn't feature for the Gunners and the Frenchman admits there is concern in the defensive department.
"We are a bit short, yes because of the injuries we have," explained Wenger.
"You sit there and you think Gibbs couldn't play because of a slight problem, Mertesacker is a bit on the edge and then we have no choice."
