Arsenal let a three goal lead slip against Anderlecht but what did we learn from the game?
Arsene Wenger's failure to bring in a centre-back this summer looks set to cost the club in numerous competitions this season.
With Arsenal limited in numbers in this position Nacho Monreal has had to slot in at the heart of defence, but he is clearly not comfortable there.
The Spaniard looked well off the pace last night, but can hardly been too heavily criticised given he has played his entire career previously at full-back.
The centre-back position is far too important to start players who are unaccustomed to playing there in and Wenger looks set to be punished for his huge oversight.
Another second place finish
After so many seasons of finishing as runners-up in their Champions League group, Arsenal should have learned the importance of avoiding second spot.
However, it looks like they are set to stay in their perennial position again this year and that means their progress in the knockout stages looks unlikely.
Where's the protection?
No team of Arsenal's quality should be throwing away 3-0 leads at home to any side, but to do so against Anderlecht is simply inexcusable.
The Belgian side are a handy outfit, but they are hardly one of the European giants and they should not have posed the Gunners any threat after the established such a vast lead.
Arsenal have far too many intricate, attacking players and not enough defensive-minded midfielders, though, and it cost them last night.
Chambers is going backwards
Calum Chambers made a brilliant start to his Arsenal career, but his performances are getting progressively worse.
The 19-year-old will have been surprised to have started so much since arriving from Southampton in the summer, but he seems to have regressed rather than progressed at the Emirates so far.
Chambers is clearly a talent, and has a big future, but it seems that taking him out of the first-team may be beneficial to both player and team.
Sanchez is carrying Arsenal
Alexis Sanchez is Arsene Wenger's best purchase in some time.
The Chilean is already Arsenal's talisman after arriving this summer and he has adapted to English football instantaneously.
His performance last night was superb as he showed numerous qualities to help his side.
Sanchez is an obvious goalscoring threat, but he is also the Gunners' first line of defence and Wenger should be building his team around the former Barcelona man.