Why is it nonsense? Fatigue has an enormous impact on the level of a player. We are the only team in the Premier League that has competed in every possible game so far, which is going to have a major impact. Liverpool, on the other hand, went out of the cups and looks to be going out of the Champions League. They have had a much easier ride.
You could see in the game today that some of them could barely run. Also, fatigue doesn't just affect physicality but also mentality, which will explain the enormous amount of poor decision making and sheer sloppiness from every player on the pitch.
I would say Ten Hag needs to give them a rest, but the squad is very limited in depth. This is why anyone legitimately believing we were in the title race were fooling themselves; we just don't have the depth for it. Even our first 11 needs changes, particularly in attack.