The German defender was at fault for Monaco’s second goal when the centre-half decided to press high up the pitch and left Anthony Martial in acres of space to set up Dimitar Berbatov.
Yannick Ferreira-Carrasco added a third from another counter-attacking move as Arsenal left themselves exposed, and Mertesacker believes this sort of naivety in their defending is what lost them the game.
“Overall it’s very disappointing for us,” Mertesacker told Arsenal’s website.
“I thought in the second half we exaggerated a bit, looked a bit over motivated at times and they got us on the break.
“That’s what we expected, but when you try to come back as quickly as possible, you can make some mistakes. That’s what hurt at the end.
“We never give up. That’s always the sense of our game, even if the expectations will be low now.
“It’s easy to write us off now but it’s just half time. A lot of things can happen over there and we will try.”
The German World Cup winner has played almost every game this season clocking up 34 appearances and scoring one goal.
But Arsenal have also had a poor defensive record conceding 29 goals in the league, more than any other side in the top-four.
They will need to improve on their defence if they wish to overturn a 3-1 deficit in the second-leg, and no team in the European Cup has been able to come back to win when conceding three away goals since 1969.
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