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Shellshocked Manuel Pellegrini was at a loss to explain West Ham's capitulation at home to Newcastle.
The haphazard Hammers found themselves 3-0 down inside an hour thanks to goals from Ciaran Clark, Federico Fernandez and Jonjo Shelvey.
They staged a belated comeback through Fabian Balbuena and Robert Snodgrass, but the 3-2 scoreline could not mask a truly calamitous display from Pellegrini's expensively-assembled side.
"It is really difficult to explain why we played the way we did in the first 45 minutes," said the Hammers boss.
"The last two games we played at home we started by pressing from the beginning, playing in the opposite half. Today I do not understand why we played so, so bad."
The Hammers are now without a win in six games, a sequence that began with a 4-0 defeat at Oxford in the Carabao Cup.
"I'm very concerned," said Pellegrini. "I think that this lapse started against Oxford and from that moment we have not won any more.
"We need to try and find the reason why this is happening."
Part of the blame must lie with West Ham goalkeeper Roberto, standing in for injured first-choice Lukasz Fabianski.
The Spaniard, a free transfer signing in the summer, was rooted to his line when Fernandez headed in his first Newcastle goal, and was painfully slow to get down to Shelvey's free-kick.
However, Pellegrini said: "Roberto has a career behind him, with big teams in other countries. I do not think today is about him.
"They scored their first from a set piece which is not his responsibility. He saved a couple of counter attacks.
"I do not think we can find one reason why we lost today. I do not think Roberto is the reason."
Roberto did make two saves to deny flying winger Allan Saint-Maximin, while Shelvey also hit the crossbar in a totally one-sided first half.
All the more galling for West Ham was the fact that, before this, Newcastle had not scored more than one goal in a match all season.
"When I look back it could and should have been four or five, that would have been a true reflection with the chances we had," said Newcastle manager Steve Bruce.
"Then at the end you think 'is it going to bite you?' We tried hard to make it bite us but I think that would have been unjust. That's what we've been after."
