"Why should he be a scapegoat?" the Frenchman asked reporters ahead of the Gunners' Premier League trip to Aston Villa on Saturday.
"For what? We've lost one game since April 1. Let's be realistic."
The World Cup-winning playmaker has come under criticism for lukewarm performances since he returned from Brazil with Germany's triumphant team.
The former Real Madrid player joined Arsenal last year for 42 million pounds ($68.78 million) but even in that first season came under fire for his frequent 'vanishing acts' during key games.
He was largely anonymous again on Tuesday when Arsenal lost 2-0 to Borussia Dortmund in the Champions League, with compatriot and former national captain Michael Ballack among the critics.
"What's happened with him? I don't know," the midfielder told Sky Sports television. "There must be something behind (the scenes) because his head is mostly down, the body language is not improving even with the world champion (medal) in his pocket.
"I don't really know what's happened over the last year since he moved from Madrid to Arsenal."
Wenger said the individual criticism was "a bit unfair" because the team as a whole put in a below par performance.
The Frenchman pointed to the recent World Cup success as a possible factor.
"It's post World Cup, he came back on the 11th of August. It takes a few months for them to get back to their best. That can happen," he said. "It's more mental than physical in my opinion."
The manager urged fans to get behind the player and support him back to his best.
"I understand that you want your best players always to make the difference in every single game and we all go through periods when we are a bit less good," he explained.
"You help. A club is a union between players, supporters and directors. You have to be united and sometimes go through periods together where it goes a bit less well."
Arsenal are unbeaten this season in the Premier League after four games, but have only won one and are seventh in the standings with six points. Villa are second on 10.
(Reporting by Alan Baldwin, editing by Pritha Sarkar)
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