The striker appeared to lack form and fitness as the Gunners lost 1-0 to Monaco on Sunday, with just six days to go before Arsenal take on Manchester City in the Community Shield at Wembley.
But Wenger is not overly concerned, saying: "He had only three weeks rest, it was not an eternity and he had to come back quickly.
“But sometimes when you come back late you want to work hard and in a game you are not as sharp.
"He's not put weight on, he's not out of shape, he's worked very hard this week and not recovered from the work he has done."
Giroud, who scored once in five World Cup appearances in Brazil, said: "It was really hard for me physically and I didn't feel really well in the game.
“But it is ok as I have this week to work because we have to be ready for City. For pre-season preparation you have to work hard and there is less than one week to be ready."