Danny Welbeck has been pulled out of the Arsenal team with an injury
Welbeck was due to start in a front three with Alexis Sanchez and Theo Walcott.
But the England striker has been forced out with illness meaning Giroud has had to step forward.
Welbeck, 26, has made just seven appearances this season with injury blighting it again.
Twitter reacts as Danny Welbeck pulls out of Arsenal's Bayern Munich clash
Tue, March 7, 2017
Danny Welbeck has been replaced by Olivier Giroud after getting ill during Arsenal's warm up to face Bayern Munich
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Tonight would have been his first Champions League appearance of the campaign.
Giroud, who replaces the forward, struggled as he led the line against Liverpool and picked up an injury himself.
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Olivier Giroud replaced Danny Welbeck in the Arsenal starting line up
But the French star is back in the firing line tonight as Arsenal try and recover from a 5-1 first leg defeat.
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Arsenal fans protested against Arsene Wenger ahead of the match after another defeat on Saturday left them out of the top four.
Wenger is out of contract in the summer and could leave ater 21 years in charge.