The Gunners’ hopes of reaching the quarter-final stage were dented after Lionel Messi’s second-half double put the Spanish champions in the driving seat ahead of the return leg at Camp Nou next month.
And Mertesacker admits Arsenal paid the price for failing to take their chances.
“You have to score at one stage against Barcelona to keep the pressure up on them,” said Mertesacker.
“They had a lot of possession but we defended well and we had to score at one point – that was the key to why we lost in the end.
“It was obviously a disappointing ending. I think we defended so well over 70 minutes and then we got punished twice. We tried so hard to cover each other [for Messi’s first goal] but someone was always a little bit late on that counter-attack.
“It was a bit naive but when you keep going for such a long time and keep up with a good team, you have to score. That [the opening goal] was a disappointing moment for everyone and we couldn’t get over it.
“We need to play better on the break and when we have the ball. We could have created more chances and shared the ball better in the final third. Obviously, we need to score. Then, we could have put pressure on them and they would have to come at us.”