LONDON — Southampton forward Charlie Austin exacted sweet revenge on a club which once questioned his fitness as West Ham lost its fourth straight Premier League game on Sunday.

Austin’s goal and assist at the Olympic Stadium gave Southampton a 3-0 victory over a team whose top official had said he wouldn’t sign the player due to the state of his knee.

In August 2015, West Ham co-chairman David Sullivan told an unofficial club podcast: “They say he (Austin) has no ligaments in his knee, who knows?”

But Austin proved his worth for Southampton, scoring his fifth goal in four games in all competitions to dispel any doubts about his physical condition or ability.

Having seen his side score more than one goal for the first time in the Premier League this season, Southampton manager Claude Puel praised Austin’s clinical play.

“He can score very nice goals and I think he scored with his first chance,” said Puel. “This shows a striker’s confidence.”

The win means midtable Southampton has eight points from six matches, while West Ham is struggling in the relegation zone with three points from six games.

West Ham conceded three or more goals for the fourth consecutive league game. Coach Slaven Bilic said the players were low on confidence but hinted he wanted players to be harder on each other.

“Heavy defeats against Watford and West Brom started the situation with the confidence,” said the former Croatia manager. “The players are good with each other — they are maybe too good with each other.”

In a first half lacking goalmouth action, Southampton always looked more likely to make the breakthrough, repeatedly threatening through Ryan Bertrand down the left.

Having escaped when Pierre-Emile Hojbjerg failed to convert the former Chelsea defender’s cross after eight minutes, West Ham was eventually made to pay.

The Saints’ chief creator, Dusan Tadic, freed Bertrand once more after 40 minutes, whose pull back was expertly slotted home by Austin.

Despite the impressive work rate of Simone Zaza up front, West Ham struggled to create any clear openings in the first half, looking most dangerous from Dimitri Payet’s set pieces.

Southampton should have doubled its lead early in the second half when Tadic found himself in space after a slick exchange of passes with Cedric Soares.

The Serb midfielder, perhaps surprised by the amount of space he’d been given in the West Ham area, shot straight at Adrian — a mistake Tadic wouldn’t repeat. Just 10 minutes later Cheikhou Kouyate gifted Southampton the ball deep inside his own half and Austin turned provider as his first-time pass released Tadic, who calmly rounded Adrian and finished in the 62nd.

West Ham attempted to rally, but the introductions of Sofiane Feghouli, Ashley Fletcher and Edimilson Fernandes failed to cause the visitors any major discomfort.

Payet’s influence grew as he fired wide from the edge of the box and Zaza saw an effort cleared off the line by Virgil van Dijk after more good work from the French midfielder.

The visitors had multiple chances to extend their lead late on and finally did so in stoppage time when Steven Davis cut back for substitute James Ward-Prowse to fire into an empty net — and not many home fans still around to see it.

There were pre-match fears over crowd trouble following violence during the league match against Watford. Off the pitch at least, the game passed more peacefully for the hosts this time. While a significant minority of West Ham fans again chose to ignore pre-match warnings over persistent standing at the club’s new stadium, the stewards appeared to take a more lenient approach.