The Gunners enjoyed a 3-0 lead before a defensive implosion of note saw the Belgian outfit get back into the game to salvage a point from their UEFA Champions League clash.
The result also means that Arsene Wenger’s men have practically no chance of finishing top of their group, with Borussia Dortmund set to take that honour.
This will again see the north Londoners take on one of the big guns in the last-16 of Europe’s elite club competition.
However, Mertesacker insisted that it was not the end of the world as they is on the verge of qualifying for the knockout phase – the Gunners need only one point from their last two fixtures against Dortmund (home) and Galatasaray (away) – and that an improvement in defence is all that is needed.
He said: “The team is very disappointed and it is a small setback for us. But we are good at bouncing back very quickly after small setbacks.
“This is a small one. It’s a good warning, that everyone should raise their game and give 10% more when it is really needed.”
The ‘small setback’ means that Arsenal are likely to face either Bayer Leverkusen, Paris Saint-Germain, Barcelona or one of the Madrid clubs in the next round.
This will be a tough ask when considering that, over the last four years, Wenger’s team have been knocked out in the last-16.
Mertesacker added: “It is going to be the same things as last year and the year before when we finished second and the draw gave us Munich.
“That is up to us. We did that. It was due to lack of defending.”