The Gunners will be keen to pick themselves up after a disappointing loss to Olympiakos in midweek.
The Gunners have been left reeling this week after suffering a Champions League defeat to Greek side Olympiakos at the Emirates, their second reversal in the competition this season.
It was a disappointing defensive display from the side, with them conceding three avoidable goals, with the third in particular a poor one given they conceded it straight after levelling the scores at 2-2.
And Mertesacker says they need to focus on their defensive discipline in future.
“We need to get behind the ball and play better defensively,” he told arsenal.com.
“We lacked discipline, even when you get a goal back don’t rush yourself. Get your discipline back and with possession you get your chances. If we are not disciplined and do not chase the ball we have got no chance.”
A good opportunity to play with possession and dynamism
They now take on Manchester United in the Premier League on Sunday, which is sure to be a big game for both sides.
While Arsenal struggled in Europe, United recovered well from an initial defeat to PSV to secure a 2-1 victory over Wolfsburg at Old Trafford.
They are also in fine form in the Premier League, having won their last three games to sit top of the table with 17 points from a possible 21. Rather than being negative, though, Mertesacker believes the clash will be a good opportunity for the Gunners to return to their usual style of football.
"I think everyone will have regrets and it is good to take that away,” he added.
“It will take a couple of days to recover and to think about Manchester United and play in a different competition, which will be a good opportunity to come back to the Emirates and play our football with possession and dynamism.”