West Ham manager Sam Allardyce believes his side must not let a "horribly wrong" offside decision derail their promising campaign after Arsenal closed in on the top four of the Barclays Premier League with a 2-1 win at Upton Park.
The Irons - who suffered a second successive defeat of the Christmas fixtures after they were beaten at leaders Chelsea on Boxing Day - looked to have got off to a flying start when former Gunners' midfielder Alex Song rifled in a 25-yard effort after just six minutes.
However, the goal was quickly ruled out by referee Neil Swarbrick, who judged three West Ham players - among them Diafra Sakho and Andy Carroll - were standing in an offside position in front of goalkeeper Wojciech Szczesny.
Arsenal recovered to net twice just before half-time through a penalty from Santi Cazorla and Danny Welbeck's close-range finish.
Although Cheikhou Kouyate reduced the deficit at the start of the second half, West Ham were unable to conjure an equaliser, despite plenty of pressure, but also had goalkeeper Adrian to thank for denying Arsenal on the counter-attack several times.
The Gunners leapfrogged the Irons to go level on points with fourth-placed Southampton, whom they play on New Year's Day.
West Ham are now sixth, but Allardyce is expecting a response when strugglers West Brom are the next visitors to east London.
"We have only lost two, not five, and we want to bounce back against West Brom," said Allardyce, whose side were beaten at home for the first time since August 30.
"I am pleased with the performance, the application, the commitment and the attitude and quality from the players. It is sad for them they didn't get the result."
West Ham 1 Arsenal 2 player ratings
West Ham 1 Arsenal 2 player ratings
Made three remarkable saves late on to stop the Gunners running away with it. 8/10
2/22 Joey O'Brien
Came in for Arsenal loanee Jenkinson and failed to impress. 5
3/22 James Tomkins
Will have nightmares about Welbeck's pace. Missed good opportunity in the first half too but did set up goal. 6
4/22 Winston Reid
Conceded the penalty after foul on Santi Cazorla. Disjointed performance. 4
5/22 Aaron Cresswell
Struggled against Oxlade-Chamberlain. 5
6/22 Alex Song
Wore the captain's armband but put in a poor performance considering his high standards this season. 5
7/22 Cheikhou Kouyate
A force to be reckoned as the game wore on and West Ham beat down at Arsenal's door. 7
8/22 Morgan Amalfitano
Tricky Frenchman always looked to get the Hammers on the attack. 6
9/22 Stewart Downing
Failed to get a foothold in the game. 4
10/22 Andy Carroll
Aerial presence had Arsenal scared, but missed glorious chance when it fell to him. 6
11/22 Diafra Sakho
Like Carroll, presence in the air worried Arsenal. 6
12/22 Wojciech Szczesny
Commanding in the air. 7/10
13/22 Mathieu Debuchy
Kouyate's header went in off French defender. Solid if unspectacular performance. 6
14/22 Per Mertesacker
Always looks a far more assured player alongside Koscielny. 6
15/22 Laurent Koscielny
Returned to the team after nearly a month out and had a busy afternoon against the Hammers. Marshaled the defence at Upton park. 7
16/22 Nacho Monreal
Had his work cut out for him but excelled today. 7
17/22 Francis Coquelin
Not overawed in first Premier League appearance since January 2013. 6
18/22 Mathieu Flamini
Still not looking 100 per cent fit. Could use a rest but growing Arsenal injury list will not allow it. 6
19/22 Alex Oxlade-Chamberlain
Rampaging run set up Welbeck's goal to make it 2-0. A constant threat. 8
20/22 Santi Cazorla
Scored his fourth goal in his last five games. Gave Adrian no chance from the spot. 7
21/22 Alexis Sanchez
Unplayable. Fought for every ball. A great performance (yet again). 9
22/22 Danny Welbeck
Great hold up play and made no mistake for second goal. Rapid pace had Hammers defence constantly on edge, but finishing touch let him down at times. 8
Allardyce continued: "If we'd been allowed that goal, we wouldn't have lost. We would have kicked on and Arsenal would have found life very difficult.
"I have not spoken to the ref because I don't see any point in confronting him when you know you're 100 per cent right and he hasn't had the visuals. You can't get anything out of telling him how wrong he's been.
"Going on what we were told at the start of the season, they got it horribly wrong. He (Sakho) wasn't in the eye-line of the keeper. He didn't touch the ball.
"Yes he was offside, but we've seen so many of those allowed as goals. I've not seen one 'not given'.
"Can you tell me what is an offside? Now I'm totally and utterly confused and bewildered. There are far too many inconsistencies."
Arsenal will again be without suspended forward Olivier Giroud at Southampton.
Gunners boss Arsene Wenger accepts the Christmas fixtures have tested a squad already depleted by injury.
"We knew before the game we would have to dig deep because we were jaded, but we did that," he said.
"When they came back to 2-1, we had five or six chances on counter-attacks that we couldn't take, but overall everybody did the job very well, from the 'keeper to up front."
Wenger added: "It's vital to re-establish ourselves in the top four. We have done it for 17 years, so we are entitled to at least believe we can do it, but it's all to do because everybody fights very hard for it."