The German centre-back was withdrawn on the morning of the match with a virus, opening up a spot for Thomas Vermaelen, who Wenger felt performed well on the day.
Mertesacker has been huge this season, especially at set-pieces where Arsenal conceded from, but Wenger could not blame the defeat on his non-appearance.
"You can't answer that, you will never know," he told Sky Sports when asked if the Gunners may have won with Mertesacker.
"Overall I believe Vermaelen had a very good game today and he was told only this morning he was coming in, and for me he led the line very well and he was all right."
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