The Gunners have had to cope with the losses of Robin van Persie and Alex Song after Arsène Wenger recruited Lukas Podolski, Santi Cazorla and Olivier Giroud this summer.
And Mertesacker admits the north Londoners did not know “where we stood” following two goalless draws in their opening Premier League games.
But after Podolski and Cazorla struck either side of half-time at Anfield to seal the three points, the Germany international is convinced Arsenal have now found their feet ahead of the international break.
“We believed in ourselves [against Liverpool] – that was the most important thing and we played fantastic,” Mertesacker told Arsenal Player.
“We had to believe a lot, because our squad has had many changes and some important players have moved. We didn’t know where we stood, but we are very happy now.
“You could see it in the dressing room directly after the match. We are very proud to play for Arsenal. We had a great opportunity here to show everybody what Arsenal is.
“We now have to focus on our strengths and that is vital for the next few weeks, to stay near the top of the league.
“A lot of challenges are coming after the international break but we are looking forward to them because we are full of confidence. We showed that against Liverpool.”
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