Mertesacker's header at the end of the first half proved enough to secure a 1-0 win at Fulham - where both sides finished with 10 men after Gunners' striker Olivier Giroud saw red in the closing moments.
It was a ninth win in 12 league games for the Gunners, who are in third place, a point ahead of Chelsea and two clear of Tottenham although both have played a match less.
That record, though, will be severely tested next weekend, when United head to the Emirates Stadium looking to close in on the title, spearheaded by the goals of Arsenal's former talisman.
Koscielny said: "I hope we can keep Robin quiet. He is a good player. I enjoyed my time playing with him, but I hope we keep the story with us."
The France defender said: "It will be a hard game for the whole team, but we are at home and we will try to get the three points.
"It will be a good game against a big team and I think they have won the title (already)."
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