The Hammers have only won one Premier League match so far and Moyes feels that the club's established stars have struggled rather than the new additions the club have spent £150 million on.
Speaking after the loss to Everton at the weekend, Moyes said: "The players who played last year are down on their level.
"We have shown signs we were getting back to it but I don’t think we did today. I need them to play at their best. I am the one who takes the responsibility and I have got to get them back at that level quickly.
"We have to keep going and keep putting the work in, the players have responded well in the last two or three years and there is no reason they cannot reach those heights again."
Moyes claimed that his players have been affected by their exploits in the Europa Conference League but the coach says the team must get used to juggling domestic and European matches.
He said: "The truthful answer would be yes but I don’t want to give it up so why would I use it as an excuse?"
Moyes felt that a draw would have been a fairer result against Everton after the 1-0 defeat and described the match as a "tight game".
The coach said: "We didn’t play well. I don’t think Everton played well either. It was a tight game. Did we deserve to win? No. Did we deserve to lose? Probably not."