Having watched the game again on Football First, then again on Match of the Day, it's clear what the problems are.
Without Alex Song, Cesc Fabregas and Theo Walcott, the 4-2-3-1 formation just doesn't work. Koscielny and Squillaci can't really play together that effectively. Manuel Almunia is a rubbish goalkeeper.
To most, these were clear before today, but that performance should put the final nails in the coffin for Arsene. There's not much he can really do about the centre back pairing at the moment, but I suspect he'll select Lehmann ahead of Almunia from here on in. As for the formation not working without the three players mentioned earlier, that won't be too much of an issue as they'll all be back for Blackburn in two weeks. Hopefully anyway.
The 4-2-3-1 is designed for four players in particular - Song, Fabregas, Walcott and Robin van Persie. Song can protect the back four while occasionally ambling forward in attack. Fabregas can pull the strings, play balls from deep or lead the attack. Walcott will receive some of those balls, and play dangerous balls into the box or get in behind the defence. Van Persie relies on the service from Walcott and Fabregas, but when they aren't there, the formation doesn't work as well.
Van Persie ends up isolated and only getting the odd chance from a cross. He has improved his heading this season, but it's not enough to hope he'll score from there. When we don't have Cesc and/or Theo, we should play Chamakh alongside van Persie. Then we can play balls aerially and along the floor to Chamakh, he can lay it off to van Persie, and we get chances.
Another problem which has been explored over and over again is how clinical we are - so many game-changing, and indeed season-changing, chances go begging. If Aaron had put that rebound past Carson it would have been 1-1 and we would have been on the front foot. Instead it was saved. I could list so many examples of where we didn't take our chances and it came back to haunt us - I won't; instead I'll point out where taking our chances helped us.
Against Chelsea in late 2010, we were underdogs despite their poor form. We didn't get many chances, but we took the important ones at the important times. We scored right on the stroke of half time having dominated - if we hadn't, our heads may have gone down and who knows how the game would have gone. Then after half time, straight out of the blocks again, 3-0, quick as a flash.
Sadly I think injuries have once again been our downfall this season - it's not our first choice team that is letting us down, it's those that back them up - Diaby, Denilson, Almunia. I'll leave Squillaci out because I feel that he's a decent player, but his partnership with Koscielny just doesn't work. We need a clear-out in the summer, of the deadwood, and then to bring in new, fresh talent who want to win. I think we should get Gokhan Inler from Udinese to back Song up and push him for a place even. That would mean we wouldn't also have to rely on Jack as much - he's played an unbelievable amount of games and it must be only a matter of time before he's burnt out.
I think we also need a right winger, someone like Theo with pace to burn. Maybe Alex Oxlade-Chamberlain, or if we're being extremely optimistic, the excellent Eden Hazard, who's been in top form for title favourites Lille. Then perhaps a Squillaci-esque centre back, only one who can play with Koscielny. However, Kyle Bartley could be the answer. We'll have to wait and see though.