Arsene Wenger knows his players can improve following their Champions League collapse but also questioned the role of fifth officials as Arsenal let a three-goal lead slip.
Bayern Munich's 2-0 victory over Roma and qualification for the knockout stages of the Champions League were overshadowed by an injury to David Alaba which may rule the two-time Austrian Sportsman of the Year out for several months.
In-form Porto sealed their place in the UEFA Champions League knockout stages with a 2-0 win over Group H strugglers Athletic Bilbao.
The result eliminates the Spaniards and sees Porto through with two games to spare.
Manuel Pellegrini was at a loss to explain Manchester City's poor performance as they were beaten by CSKA Moscow to all but end their Champions League hopes.
Chelsea manager Jose Mourinho admitted his side paid the price for a lacklustre first half as they were held to a 1-1 draw at Champions League minnows Maribor.
Lionel Messi equalled Raul's record of 71 Champions League goals with a brace to earn Barcelona a 2-0 win over Ajax in Group F.
Fernandinho and Yaya Toure were sent off as Manchester City crashed to a damaging 2-1 loss to CSKA in the Champions League.
The pair were both dismissed in the second half as City struggled to get back into a Group E contest they were anxious to win to revive their campaign.
Liverpool boss Brendan Rodgers needs to explain his team selection against Real Madrid to the fans, UEFA president Michel Platini said on Wednesday.