Lionel Messi netted a second-half brace as Barca placed one foot in the Champions League quarter-finals with a 2-0 first-leg victory at the Emirates Stadium last night.
The Gunners had matched the reigning European champions for large spells, with striker Giroud missing a good chance early in the second period.
Twitter was rife with unflattering comments for the Frenchman after his botched reverse pass then broke down a promising counter-attack by the hosts, just minutes before Messi opened the scoring.
The jet-heeled Walcott had replaced Alex Oxlade-Chamberlain on the wing shortly after the break, but Southampton boss Koeman feels that he should have been deployed centrally from the start.
“I’m surprised with the line-up; with Giroud up front,” he told BT Sport.
“You need fast people for the counter attack. Remember the Arsenal game at home against Bayern, Walcott was the striker.”
Wenger also came under fire for his decision to introduce Mathieu Flamini, who conceded a penalty within minutes of his arrival as the Catalans doubled their advantage.