The Gunners suffered a 3-1 defeat in the fourth round, with West Ham scoring all three of their goals inside the opening hour.
Ben White's own-goal put them ahead before Mohammed Kudus and Jarrod Bowen struck after the break.
Mikel Arteta unloaded his bench to try and turn the tide, handing Declan Rice a first appearance back at his old club, but they only struck when Martin Odegaard scored with the last kick of the game.
Simon Collings was at the London Stadium to see Arsenal in action...
Aaron Ramsdale 6
Handed a first appearance since the third round in September. Unlucky for the West Ham opener as appeared to be fouled by Tomas Soucek. Made a great save to deny Bowen. Beaten by a deflection for the third.
Ben White 5
Gave West Ham the lead when he headed into his own net from a corner. Was below his usual level all night.
Jakub Kiwior 6
Did better than most of the defence, especially considering he was asked to play as the right centre-back.
Not quite as sharp playing out from the back as usual. Went close to scoring with a header in the first-half.
Oleksandr Zinchenko 5
Involved in plenty of the play, as you'd expect, and played a couple of great balls forward. Beaten too easily for Kudus' goal, though, and taken off shortly after.
Fabio Vieira 5
Felt like he was bullied by West Ham's physical midfield. Started the season so well, but form has dipped recently. Did setup Odegaard's late goal.
Back in the team and charged with playing at the base of midfield. Fine in possession but defensively his lack of pace was an issue.
Kai Havertz 5
Went close to scoring on two occasions in the first-half with headers. Faded after the break, though, and was anonymous at times.
Reiss Nelson 6
Looked lively and was one of Arsenal's better players in the first-half. Put in a number of decent set-pieces.
Eddie Nketiah 5
Spurned a good chance towards the end of the first-half when picked out by Zinchenko. Barely had a sniff in the second-half.
Leandro Trossard 5
Was fairly quiet and struggled to get into the game. Feels like he is best used as an impact player off the bench.
Takehiro Tomiyasu (Zinchenko 57') 6
Solid, as he has been recently, and should start at Newcastle on Saturday.
Declan Rice (Jorginho 57') 6
Not the homecoming he would have wanted and couldn't turn the tide.
Bukayo Saka (Nelson 66') 6
Brought on with around 25 minutes to go and was livelier than most.
Gabriel Martinelli (Trossard 66') 5
Also came on with Arsenal 3-0 down. Was fairly quiet.
Martin Odegaard (Nketiah 80') 7
Off the bench and scored with the last kick of the game.
Subs not used: Hein, Saliba, Cedric, Elneny