Mertesacker was less of a regular during his first 12 months in English football but Wenger believes he was simply a victim of circumstance.
"To his defence I must say last year he arrived on August 31, and had no pre-season, had an ankle injury and had to adapt to a completely new world," said the manager to arsenal.com.
"Centre back is difficult for people who come from abroad, it was a bit of a shock for him, but this season he has come back very fit, well prepared, and of course knew what the expectations were for him and did extremely well.
"This season especially, as you [the media] have noticed, he has been outstanding.
"The criticism was always completely unjust," added Wenger. "It was overboard, we always say you have to deal with that."