WEST Ham captain Jarrod Bowen nicked all three points as Southampton suffered that familiar sinking feeling.
The travelling fans taunted St Mary’s by singing “Oh when the Saints go marching down” after Bowen prodded in just before the hour mark.
And for all the signs of improvement in the first half in charge of new boss Ivan Juric, it is hard to see any hope for Southampton.
Especially if they keep failing to take the kind of chances they had in the opening 45 minutes.
The Hammers, hit by a serious injury to Lukasz Fabianski, made three substitutions before the second period began.
They were almost reduced to 10 men soon after, when referee Lewis Smith showed Guido Rodriguez a straight red card for a challenge on Kyle Walker-Peters.
But Smith downgraded it to a yellow after being summoned to the pitchside monitor by VAR and Bowen made sure West Ham took advantage of the reprieve.
And if that was terribly hard on Southampton, the home side did not do enough to trouble the visitors as another winnable game slipped away from them.
West Ham almost took the lead inside five minutes when Aaron Wan-Bissaka was allowed to run and run before pulling the ball back for Carlos Soler.
But the Spanish midfielder, in for the suspended Lucas Paqueta, hit the crossbar with a rising shot.
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From then on the home team had the better chances and should have taken the lead.
Kyle Walker-Peters rescued an overhit Yuki Sugawara cross by volleying it back into the middle. But Onuachu put a free header wide of the post.
The Nigerian teed up Lesley Ugochukwu for a shot that was blocked by Guido Rodriguez before Mateus Fernandes failed to convert another golden opportunity.
Adam Armstrong’s cross found Fernandes all alone but from close range he could only send his header straight at Fabianski.
The West Ham goalkeeper made two more saves, from a Walker-Peters shot and an Onuachu header, and was then Pole-axed at a corner by Saints defender Nathan Wood, who was booked.
Fabianski was taken off on a stretcher and Lopetegui had to make his second substitution, having already seen centre-back Maximilian Kilman leave the field.
Hammers skipper Bowen was doing his best to alter the balance of the game but even he had only one blocked shot and another that went wide to show for his efforts.
But the visitors nearly broke the deadlock in the dying minutes of the half.
Mohammed Kudus got to the byline and crossed beyond the back post, where Wan-Bissaka hit a volley that went well wide in the end.
There were 10 minutes of added time following Fabianski’s injury and they yielded two more changes for the home team.
Substitute Jean-Clair Todibo stopped Onuachu from getting on the end of the first but when the big man met the latest Sugawara centre, his header was too close to Alphonse Areola.
By Andy Dillon
All that has been achieved under David Moyes has made West Ham forget that they have by far the worst home ground in the entire Prem.
A stadium totally unfit for staging regular football matches.
One that can work on big European nights when the fans have electrified their voices in the nearby Carpenters Arms before kick-off.
But one which drains your soul during routine home games against the likes of Burnley or Everton.
A home ground where giant trampolines separate fans from the action.
With temporary seating and scaffolding trying to bring the players and crowd closer.
When West Ham are playing badly but winning, issues with the rented stadium are pushed aside.
If they are playing badly and losing, the whole festering sore surfaces and the frustration will be aimed once more at chairman David Sullivan and vice-chair Karren Brady.
It wasn’t long ago that fans were invading the pitch, that 8,000 protested against the owners, that Sullivan was hit in the eye by a coin thrown from the crowd. Moyes has been an effective human shield with his consistent, if not entirely pretty, brand of success.
This is what incoming boss Lopetegui has to contend with.
West Ham midfielder Rodriguez came as close as you can to being sent off shortly after the break.
Referee Lewis Smith showed a straight red card for the Argentinian’s challenge on Walker-Peters.
But having been called to the monitor by VAR and seen Rodiguez win the ball before cleaning out the Saints defender, Smith reduced it to a yellow,
And that sinking feeling deepened just before the hour. After a game of head tennis, Edson Alvarez, who had been introduced at half time as the Hammers’ third sub, crossed the ball deep.
Tomas Soucek nodded it back towards the centre, Fullkrug helped it on and Bowen prodded in from close range.
The goal deflated the Saints on and off the pitch.
Fernandes put a shot narrowly wide but the home side barely troubled Areola.
Crysencio Summerville flashed an effort wide shortly after coming on for Fullkrug and Kudus fired into the side netting.
There was little action in either penalty area before Onuachu sent another header wide.
Substitute Tyler Dibling fired over the bar in the fifth of seven minutes of added time, and Onuachu’s final header of the day drifted off target in the last.
