Giroud, who had come on for Yaya Sanogo just after the hour, made sure with a late double - the second following a sublime passing move from the resurgent hosts.
"It was important because he was on the end of things and shows that, when a player comes of the bench of that quality, it can make a difference," Wenger said.
"He was on the bench for what reason? He has played many games. For the rest, he has come on and showed he can score goals when he comes on. Yes (he needed a rest). Ozil was as well on the bench."
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