The Gunners needed only a point against Napoli on Wednesday night to be sure of winning Group F and with it securing seeding for the knock-out stage.
However, after their 2-0 defeat at the Stadio San Paolo - where midfielder Mikel Arteta was sent off in the second half - and a late winner for Borussia Dortmund in Marseille, Arsene Wenger's men are now likely to face one of Europe's leading teams.
They could be paired with Barcelona, Bayern Munich, Real Madrid or Paris St Germain when the draw is made on Monday in Nyon, Switzerland.
Germany international Mertesacker, however, sees no reason why the Barclays Premier League leaders cannot raise their own performance levels once more, as they did with superb away wins over both Bayern last campaign and then, earlier this season, Dortmund.
"We got Bayern Munich last year and nearly went through. If you want to achieve something in the Champions League, you have to take every opponent seriously and I think it is better for us when we have a good lot, when there is a draw that we can concentrate on," Mertesacker said.
"When you look at the teams that finished top, if you pick one out, for me, I don't mind.
"You never know what can happen with a good draw and a serious opponent, you can get so much out of a defeat or a disappointment.
"I hope that we learn a lot from Naples and we are aware of the next opponent."
Arsenal now face a test of their domestic credentials when they travel to Manchester City on Saturday lunchtime, before hosting Chelsea on December 23.
Mertesacker said: "It is always difficult especially now after 15 games in the league and the Christmas period is coming.
"You think you have a good chance to achieve something this season but you have to keep your momentum and that is really important.
"Now everybody is pulling a bit, questioning our group, and if we can maintain that level so we have to stay together and think that 'yes, we are strong enough'."
Goalkeeper Wojciech Szczesny feels Arsenal can lay down a marker at the Etihad Stadium on Saturday, having missed the chance to open up a seven-point lead last weekend when they were held to a 1-1 home draw against Everton.
"City have scored a lot of goals at home but our defensive record in the Premier League has been very, very good this season as well, so we hope we can keep that going and keep our run in the Premier League going to secure our place at the top of the table," the Poland international said.
"However, we don't look at our position in the table.
"We know we are top and people are talking about a title challenge but we will take it game by game, focus on Man City, get a good result and if we do that we will be in a very good position."