Wojciech Szczesny has said a "poor" five-minute spell at the Liberty Stadium cost Arsenal as Swansea came from behind to record a 2-1 Premier League victory on Sunday.
The Gunners led through Alexis Sanchez's strike on 63 minutes, but Gylfi Sigurdsson equalised with a wonderful free kick with 15 minutes to go.
The impressive Jefferson Montero then crossed for Bafetimbi Gomis to head the winner just three minutes later, and Szczesny said a lack of focus from Arsenal had allowed the Swans back into the game.
"For the majority of the game we looked solid at the back and we looked dangerous," the goalkeeper told the club's official website. "We had a poor five-minute spell, and at this level that costs you.
"Until they scored their first we were good. Then after they scored the second goal we looked dangerous again, so it was the five minutes that cost us the game.
"It's something that we need to focus on because it's been two games where short spells lacking discipline and focus have cost us.
"It could have gone either way, but it didn't. You can't just hope that the next one will be different and we'll get a bit of luck -- we need to focus for 90 minutes and do our jobs properly, offensively and defensively. Then things will change."
Sunday's defeat leaves Arsenal 12 points behind behind league leaders Chelsea, with Arsene Wenger suggesting the Gunners need to be "more realistic" about their title chances if they keep making costly errors.
Per Mertesacker, meanwhile, says the club must start playing "serious football" to get their season back on track after Sunday's shock loss.
"The game was completely in our hands, especially in the second half when we scored," he told the club's official website. "Then we have to play serious football and not lose the ball but they got us on the break, and that's completely unbelievable in our situation.
"We have to play simple, even if we are 1-0 up -- even more then. We played through the middle but we need to keep possession and keep patient, something that we did all the way through until the 60th minute and we had the game in our hands completely.
"That's what I mean by serious football. When you get such a result [like against Anderlecht] normally you come back quickly and after such a setback normally we bounce back immediately.
"But that was not the game and we have to take the [international] break now really seriously and work hard on the issues.
"We played our game for 60 minutes with patience, seriously, and they didn't have a proper chance. Then we scored and we thought we didn't need to keep possession or patience, and then we struggled.
"That was our fault and we did the same against Anderlecht. We have to think about it quickly. We have good ideas on how we can extend the good 60 minutes into 90 minutes.
"It's absolutely necessary in the next couple of weeks. We struggled a bit but we have great potential and you can see that. Now we have some time to think about the few issues that cause trouble because we made mistakes in areas we have to avoid."