The German international has helped Arsenal keep three clean sheets in seven Premier League matches so far this season.
He said: "It can be the difference, it is crucial for us that the defence is winning more than just games. We must try to keep that in our mind the whole year.
"We have not conceded a lot of goals during games this year, most have been from set pieces, we just need to keep concentrating.
"The manager told us before the start of the season that we have a lot of good centre-backs and is convinced we can make the team stronger and stronger because we have a lot of those good players in every position."
Mertesacker returned from a bout of flu to help Arsenal beat West Ham United 3-1 after going a goal down this weekend.