The Germany international moved to England in the last transfer window, but has been lamented in the press for a number of mistakes since arriving in North London.
Despite this, the French coach believes his lofty centre back with prove the doubters wrong.
“I am very happy with Per,” Wenger told Mirror Football.
“The criticism in the papers has been very harsh, but when you play for a big team, you have to accept that.
“I also think it will take him some time. He came here not completely fit. Now he is getting stronger and stronger.
“I felt on Saturday he had not recovered from the Holland game [in midweek] and looked tired. But he has recovered now and will play against Dortmund.
“Of course he is under pressure and scrutiny. But you don’t feel sorry for a situation like that because that’s what happens at a big club,” he stated.
Mertesacker is in contention to face an old Bundesliga foe in the form of Borussia Dortmund on Wednesday night in the Champions League, in which Arsenal can ensure qualification to the knockout rounds.
By Gareth McKnight