Mertesacker has been a virtual ever-present since signing from Werder Bremen just before the transfer deadline and Wenger believes he brings qualities that have gone unrecognised.
"I feel people have been harsh in their judgement," Wenger told arsenal.com.
"I like Per very much, because he is a very intelligent boy and reads the game very well. He has a calming influence, a commanding influence.
"Because he is so tall, sometimes if he looks less impressive in one situation, people make a definite negative judgement.
"But he grows stronger in every game and we watched him again in Germany against Holland in midweek, and he had an outstanding performance. I am really happy with him."