Although they were easily beaten, the German defender expects a much better showing when the Premier League leaders head to fourth-placed Manchester City on Saturday.
"It's a strange atmosphere at the moment because we lost, by far, especially in the second half," the centre-back told Sky Sports.
"We were always on the back foot. We took a little break today, but we'll be back on Saturday."
Mertesacker added: "We gave 10 per cent less mentally because we were always thinking we were through.
"I think we were lucky today but we deserved (to go through) because we played a good group stage. Now we concentrate. We're second in the group for the draw next week."
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