Southampton manager Ralph Hasenhuttl said the goals his side conceded against West Ham United were an "absolute joke".
The Saints lost 3-1 at the London Stadium despite having 66% possession.
"If we played like this against a team that is in a relegation battle, then you are also very quickly back in the relegation battle, so pay attention," Hasenhuttl said.
"For me, simply not good enough."
Southampton slipped to 13th after Crystal Palace's win at Brighton earlier on Saturday and are nine points clear of the relegation zone.
They have won four of their eight Premier League games in 2020.
"We trained the whole week, we know what will come and cannot defend this ball," the Austrian said. "If you want to take something you cannot give such simple goals.
"The opponent had showed that they want to fight for everything and we didn't seem to be serious enough.
"We are only a winning team if we defend like they did today, and that was the big difference today."
Hasenhuttl added that the players "have a problem" if they think they are already safe from relegation.
'We can't act like he's the saviour'
The win was just West Ham's second since David Moyes returned to the club on 30 December for this second spell as manager.
After beating Bournemouth 4-0 on New Year's Day in his first game back in charge, the Hammers have drawn two and lost five in the Premier League.
"We certainly needed it, but I said before the game - good performances lead to results," the Scot said.
"We had a good performance against Liverpool, the players have been getting better, we've had really tough games, and we took it on today."
The Hammers only had 34% possession but they created more chances than Southampton and should have won by a larger margin.
"There was a lot of good things and a lot of good performances," Moyes added.
"I've had a few boys from the Championship [like Jarrod Bowen] - I hope I've got an idea what I'm doing with them.
"He showed exactly what he showed in training, that he can score, but we can't bring him here and act like he's the saviour of West Ham."