They were beaten 2-1 in Croatia by Dinamo Zagreb.
Olivier Giroud was sent off in the first half after picking up two yellow cards.
Wenger admits his side weren't at their best – but says the referee wasn't either.
Despite their early season problems in the Premier League, Chelsea were the only English side to win their opening group match in the Champions League.
They put recent performances to one side in easing past Maccabi Tel Aviv 4-nil at Stamford Bridge.
Chelsea manager Jose Mourinho wants all English sides to do well in Europe.
