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 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 St Stephen's 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.
West Ham are now sixth, but Allardyce said: "We have only lost two, not five, and we want to bounce back against West Brom. If we'd been allowed that goal, we wouldn't have lost."