Arsenal manager Arsene Wenger believes that striker Olivier Giroud should not have been sent off in the Gunners' 2-1 away defeat against Dinamo Zagreb.
The Frenchman was shown two yellow cards in the first half.
But the 65-year-old says the referee's decision to show a second yellow card was harsh.
"It is not the way we wanted to begin the competition," Wenger told BT Sport.
"We were a bit unlucky. We faced a Zagreb team who were committed.
"We gave it all until the end but unfortunately couldn't come back.
"The first yellow for Olivier Giroud is a foul for us and the second is an accident. They got away with obvious fouls many times.
"We have enough games to come back. We focus to win our next home game and I'm sure we can beat Zagreb at home."