I think it's more the fact that they are a unit, playing exactly the same way as they have done for the last 2 years. They know their jobs inside and out, and know exactly who is where and what they are responsible for. Far too much emphasis is being put on tiredness imo. It's not like we are fading in games, we look confused and disorganised from the off.
tiredness is not just running around....it shows up mentally and if you look at the wayward passes, poor play on a basic level from, for eg, Eriksen, Dier and Dembele, it illustrates fatigue. Several of the commentators have been referencing that and Poch has talked about the lack of time between WC SF and start of season. Seems strange to minimize an obvious data point.