Seldom have promoted teams entered the Premier League with as much confidence as Southampton did this year. Two successive promotions under Nigel Adkins, a healthy transfer fund and the fact that Norwich, Swansea and Queens Park Rangers all avoided relegation last season led to declarations from St Mary's that they would seek to take the top flight by storm rather than just cling on for survival.

So far they have been true to their word and scored a lot of goals – but amassed few points. Confidence is starting to crumble. Saints' noble ambitions are being undermined by grubby defending and unless that changes they will not emulate last season's promoted trio but re-enact the entertaining but brief visit of Ian Holloway's Blackpool to the Premier League two years ago.

This heavy defeat by West Ham United is the most damaging of the campaign to date. Most of the previous ones were dished out by the established powers but this one came against a club that finished below them in the Championship last season. And it was deserved, not because West Ham were excellent – far from it – but because Southampton's blundering was basic.

After a goalless first half Mark Noble opened the scoring with a 40-yard freekick that bounced into the net and the midfielder struck again from a penalty after a clumsy handball by José Fonte; Kevin Nolan and Modibo Maiga scored after defenders were outmuscled embarrassingly. Adam Lallana's fine strike for Southampton, and some tidy midfield play by the visitors, counted for nothing.

"We couldn't get anywhere near Southampton last year … but the difference between us and them in the second half shows how far we've come," said the West Ham manager, Sam Allardyce, who could also have suggested that it shows how far Southampton have failed to come. "I think the players we have brought in and added to the squad have delivered."

Southampton's recruitment has been more questionable. They spent most of their summer budget on attacking players, including £12m on the winger Gastón Ramírez and £6m on the striker Jay Rodriguez. In comparison they skimped on defenders. The new right-back Nathaniel Clyne has done well, although most of his best work is going forward, but no one else has consistently defended to the level required.

Adkins has tinkered with the make-up of his back four – partially because of injuries – but all permutations have proved porous. The centre-backs Fonte and Jos Hooiveld were shaky against West Ham, while Maya Yoshida was as flimsy at left-back as he was at centre-back during the 6-1 defeat at Arsenal. Adkins has also tried three different goalkeepers – with Artur Boruc, signed as a free agent a month ago, making his debut here. The manager has also adjusted his formation in a bid to provide extra security to his rearguard – switching from the 4-4-2 he used over the last two season to a 4-3-3 for this one.

Lallana and Morgan Schneiderlin have made more interceptions than anyone in the division and Schneiderlin is only behind Noble in terms of the amount of tackles made – but that has still not been enough to prevent Saints from self-destructing. "Individually some players are maybe getting found out a little bit, but it's a collective thing at the football club," said Adkins. "We've all got to stick together and we've all got to work hard."

That work does not entail changing philosophy, said Adkins, just applying the current one better. "There's a principle on how the football club wants to go," he said. "Who's to say looking at alternatives is going to more beneficial for us? Our style of play dictates we're scoring goals. But we're not keeping the ball out of the back of the net. We've got to sit down, analyse and look at what's going wrong." It may simply be that the defenders are not good enough.

Man of the match Mark Noble (West Ham).