The German defender has again called for consistency in the Arsenal ranks.
The two may not be title contenders this season, like they were in the past, but the rivalry between them still exists strong - even if times have changed some what.
For Arsenal it is a game they need to win. They are currently struggling for results with recent games against Swansea City and Anderlecht not going to plan.
Per Mertesacker has been discussing that form in recent weeks, and is now making it clear that they must remain consistent for 90 minutes to claim victory against Manchester United later.
"We had a consistent 60 minutes twice and we have to extend that to convert it into a decent performance for 90 minutes. We lacked something after 60 minutes, just small mistakes, and that turned it completely around.
"We thought that we were secure and safe after 60 minutes and we lost our head twice. That’s really unusual because when there was a small setback last season we came back even stronger.
"At the moment we are really spot on for 60 minutes and we cannot afford to lose our heads again, we have to be consistent over 90 minutes and we need to get that in our mind."
Arsenal have been conceding late goals far too often this season and Mertesacker knows that against Manchester United that will likely be punished with defeat.
Time will tell if his team-mates heed his warning.