Declan Rice's absence is to blame for a number of heavy West Ham defeats this season, according to manager David Moyes. (More Football News)
West Ham sold captain and influential midfielder Rice to Arsenal last July for a club-record fee rising to £105million with add-ons.
The Hammers have seen results nosedive since the turn of the year, most recently crashing to a 5-0 defeat away at Chelsea on Sunday.
That was the sixth time West Ham have conceded four or more goals away from home this season, and their fifth loss in six away games overall.
Asked at his post-match press conference exactly why his side are on the end of heavy losses so frequently of late, Moyes said: "Declan Rice."
The Scotsman, due to be out of contract next month, added: "You get the best midfield player in the country, protecting, making sure the moments and times you limit maybe 50 per cent of the attacks.
"It makes you a much better defensive team when you get that. We've lacked protection in front of the back four; we've lacked good enough defending; we've not been good enough on those things in many games.
"You've got to be careful. You're talking about a team sitting in a really, really strong position. We've had a couple of bad days away from home, which we have to try and eradicate and make better.
"I'm trying to put a bit of mental toughness when we need it. The teams I normally prepare would normally have it. You could always lose the way you lose."
West Ham have won just one of their past nine Premier League games, seeing them slip down to ninth place and now out of the top-six running.
United have also failed to keep a clean sheet in any of their past 16 in the competition - only the second time they have done so in a single Premier League season.
The poor run of form, which also includes elimination from the Europa League at the hands of a strong Bayer Leverkusen side, has raised doubts over Moyes' future at London Stadium.
Amid suggestions that former Real Madrid and Spain boss Julen Lopetegui has already been lined up, Moyes reiterated he will wait until after the season to hold talks over his future.
"I'm going to talk to the board at the end of the season, so we’ll do that then," he said.
