The Red Devils slipped to their third defeat of the season against the Baggies, as goals from Morgan Amalfitano and Saido Berahino gave Steve Clarke's side the win at Old Trafford.
Moyes told reporters: "There were lots of things that disappointed me. I don't think the midfield was just the one department where you'd say we were lacking. I think it would be unfair to say it was any [specific area].
"It was the whole thing, we were lacklustre in nearly all the areas. So we've got to try and work on it and improve. We've just not played that well. We should be scoring more from open play."
United are 12th in the league table after six games this season.