The French striker was substituted on the hour-mark after failing to find his feet in front of goal and missing a gilt-edged chance to pull a goal back for Arsenal in the second half.
Geoffrey Kondogbia’s deflected 25-yard strike gave the Ligue 1 side the lead in the first leg of the last-16 tie and Dimitar Berbatov then finished off a counter-attack from 12 yards to make it 2-0 in the 53rd minute.
Wenger’s men pulled a goal back when substitute Alex Oxlade-Chamberlain curled home in the 90th minute, but Yannick Ferreira Carrasco scored in stoppage time to put Monaco in charge of the knockout clash.
Asked about his decision to substitute Giroud, Wenger told his post-match news conference: “I felt he had an off night tonight.
“He missed easy chances and it looked like it was not one of his best days.”
Wenger also defender Oxlade-Chamberlain after the midfielder was caught in possession in the build-up to Monaco’s third goal.
“He’s very disappointed but it can happen that you lose the ball 70 metres from goal,” he added.
“It doesn’t mean you have to concede a goal, you have defenders behind you.”
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