West Ham boss David Moyes admits losing Declan Rice last summer was a major blow.
Rice was sold to Arsenal for 100m before the start of the season.
Asked about a reason for their heavy defeats, Moyes said: "Declan Rice.
"When you get the best midfield player in the country, protecting [and] limiting 50% maybe of the attacks.
"When you get that, it makes you a much better defensive team. We've lacked protection in front of the back four."