Personally, I think it's just as much as of a problem in big games, especially when you consider that Conte likes to play on the counter against the big teams. Good counterattacking opportunities are rare against big teams so every single one needs to be executed to perfection, something Willian doesn't do well IMO. His understanding of picking the right pass in transition is really poor so his dribbling is the only weapon left which (when he's not in good form) can be just as shit as his decision-making as we've seen in the past few games.