Liverpool manager Jurgen Klopp has compared the Nations League to world heavyweight champion Anthony Joshua having to box “every second night”.
The Reds boss called the competition “senseless” before the international break, citing added strain on players.
Nations can be promoted and relegated in the new Uefa format, which replaces friendlies with competitive games.
“I like competition but at one point somebody has to step back and think, they are players,” Klopp said.
“If they don’t perform I am angry, so how can we make sure they perform?”
Liverpool, who are third in the Premier League and travel to Huddersfield on Saturday, had players including Virgil van Dijk, Joe Gomez and Jordan Henderson in Nations League action.
But their casualties were Mohamed Salah and Naby Keita, who sustained muscle injuries in Africa Cup of Nations qualifiers, and Sadio Mane, who broke a thumb training with Senegal.
The Nations League offers a route to qualification for the 2020 European Championships. Klopp believes it is a “good idea” as a concept but one that does not work in practice because of a lack of “space” in the calendar.
He added: “At one point we have to think do we want opera every night, or every two months?
“You don’t want to see Anthony Joshua fighting every second night. That’s not possible. In no other sports – in American football you have a summer break that is nearly longer than our season has.
“In basketball they have a summer league. Only in football everyone wants big competitions where you can get relegated, promoted, playing for something.
“The Nations League is a good idea but do it in another sport because in football there is no space for it. I realise that I could have told my coffee machine because nobody is really interested [in my opinion].”