Arsenal manager Arsene Wenger claimed his side were "a bit unlucky" after a 2-1 defeat by Dinamo Zagreb in their Champions League opener.
Wenger made six changes for the game after Saturday's Premier League win over Stoke.
Striker Olivier Giroud was sent off after 40 minutes and Wenger also blamed "exceptional circumstances".
"I believe the first goal is offside and after the sending off is very harsh," said Wenger.
'I don't understand the referee'
Arsenal went behind to an Alex Oxlade-Chamberlain own goal before Giroud's dismissal for picking up two bookings.
The France international was cautioned for dissent by Romanian referee Ovidiu Hategan before a second yellow card for a foul on Ivo Pinto.
"We were not at our best, but nor was the referee," added Wenger.
"The first yellow is Giroud's fault because he protested, even if it was not a foul against him.
"The second yellow card, he is unlucky to touch the guy - he did not see him. It was accidental completely, but it happened.
"I don't understand the referee. It has to be on purpose, a foul on purpose. Then the referee should have given many yellows."
'It didn't work'
Wenger dropped keeper Petr Cech, left-back Nacho Monreal, midfielder Francis Coquelin and forward Theo Walcott to the bench for the game, while midfielder Aaron Ramsey and right-back Hector Bellerin were not in the squad.
"When you don't win the game you have to look at yourself and think you haven't got it right," said Wenger, whose side travel to Chelsea in the Premier League on Saturday.
"I don't believe the players who came in had a bad game but it didn't work.
"I have to analyse that well. It is very difficult to give a definite answer straight after the game."
Wenger remains confident
Arsenal began the Premier League season with a 2-0 defeat by West Ham and have again made a losing start to their Champions League campaign following a 2-0 defeat by Borussia Dortmund this time last year.
However, Wenger - who did not sign one outfield player during the summer - is still confident his side can qualify from a group currently topped by Bayern Munich after their win over Olympiakos.
"Overall, I believe we have problems to start in every competition this season. But I am confident we can put that right in the next five [Champions League group] games."
He added: "Mathematically, we can qualify without any problem if we produce our performances that we want at home."