Arsene Wenger said Arsenal had paid for their lack of defensive experience in the 4-3 home loss to Liverpool on Sunday, with the late return of key players after Euro 2016 creating "an impossible situation."
But Wenger said there was no reason to panic after his team were booed off at the final whistle, adding that any new additions would have to be the right ones.
Arsenal led 1-0 at the Emirates but, having been level at 1-1 at the break, were torn apart during a 15-minute spell as Liverpool went 4-1 in front before they pulled two back through Alex Oxlade-Chamberlain and Calum Chambers.
"We played a very good first half and I think we were very unlucky to concede a goal just before half-time," Wenger told Sky Sports.
"We didn't recover really from that, and after that we paid for a lack of experience at the back and the fact that some players are not physically ready to last 90 minutes at that level.
"But overall it's disappointing, because in the first half you think we should come out with at least one goal [in front] for us, and 10 minutes later you are 3-1 down. It was difficult to recover.
"We did fight until the end, but we lacked stability a bit."
Arsenal played 21-year-old Chambers and 20-year-old Rob Holding at centre-back, with Per Mertesacker and Gabriel Paulista injured and Wenger opted against rushing back Laurent Koscielny.
The France defender, striker Olivier Giroud and Mesut Ozil were all rested after Wenger gave them extended holidays following the European Championship.
Wales midfielder Aaron Ramsey did start, despite only playing one preseason game, and was substituted in the second half with what appeared to be a hamstring injury.
Alexis Sanchez, whose country won the Copa America in June, was also given an extended break as he recovered from an ankle injury but started up front and looked short of full match fitness.
"We pushed players today, like Aaron Ramsey," Wenger told the BBC. "It is an impossible situation to get players back from the Euros and prepare."
But he said any new addition in the transfer window "has to make sense and to be quality," adding: "There are many reasons [for the Liverpool result] that you have to analyse well, but it is as well not to panic.
"I've said many times we are working very hard to get one or two players in."
There were boos from home fans at the end of the game, and Wenger said: "Of course it's not pleasant, but we have shown in the past that we have the mental strength to deal with that and bounce back."
The Frenchman, in the final year of his contract, added: "I am always under pressure because I want to win every single game. And that is part of the game as well."