The Arsenal centre-back says a dressing room inquest followed the defeat in the north London derby which has inspired the side's recent solidity away at Bayern Munich and Swansea
Arsene Wenger's defence were culpable for both goals conceded against rivals Spurs, with Mertesacker and central defensive partner Thomas Vermaelen both singled out for criticism for lax concentration.
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"Sometimes as a player you have a different view and you can't believe what we did as a defensive unit," the Germany international told reporters.
"Sometimes you need to learn from games and that was very important for us. There was a lot of room for improvement after the Tottenham game – we saw a lot of things on television. Sometimes we didn't choose the right direction and we just lacked a bit of defensive work as a unit."
The Gunners have been criticised for their defensive displays all season long despite the introduction of former stalwart centre-back Steve Bould as Wenger's new assistant manager at the start of the season.
"There were a lot of situations where there was no pressure on the ball and the back four was not prepared for any balls between us," Mertesacker continued.
"To cover each other, that was the most important thing we learned."
Arsenal's convincing victory at Swansea, courtesy of two late goals from Nacho Monreal and Gervinho, kept up their chase for a place in next season's Champions League and they now lie only four points behind fourth-placed Tottenham with a game in hand.