West Ham manager Sam Allardyce defended his decision to switch formation as his side fell to a 1-0 defeat at Championship leaders Southampton.
The Hammers boss changed from the 4-4-2 formation employed in Saturday's 4-0 demolition of Blackpool to a more defensive 4-3-3.
But Jos Hooiveld's header proved enough to end second-placed West Ham's unbeaten away record and stretched their lead at the top to five points.
On-loan Celtic defender Hooiveld powered home the winner on the stroke of half-time in front of a St Mary's record crowd of 32,150, but they were put under immense pressure by the Hammers, who hit the post late on through Mark Noble's cross-shot.
Allardyce said: "We knew Southampton would be dangerous coming forward and I wanted to nullify their attacking strengths.
"The first half was very tight and we created some chances so were disappointed to concede right before half-time.
"This is not an easy place to come and on another day we would have got something out of it."
West Ham suffered another blow when Matt Taylor limped off to be replaced by Frederic Piquionne with what looked to be a recurrence of a calf injury.
