The Reds' 3-0 defeat by Napoli at Murrayfield was their fourth successive preseason game without a win, BBC Sport reported.
"I don't want to find excuses before we play [City] but we had a completely different preseason," said Klopp.
"We played it without six players. From a freshness point of view, the games came at the wrong moment."
Liverpool has been without goalkeeper Alisson, and strike trio Roberto Firmino, Sadio Mane and Mohamed Salah because of international commitments, while Naby Keita and Xherdan Shaqiri have been injured.
"Have we had the best preseason of our lives? I cannot say that with the results, but the sessions were really good," said Klopp, whose side plays Lyon in a friendly on Wednesday before the game against Premier League champion City at Wembley.
"The problem is all the games are in public so everybody sees what we're doing. But we have to prove ourselves during the season, not in one game."
Keita and Shaqiri have resumed training, while Alisson, Firmino and Salah will returned to the squad on Monday.
Mane is still absent after Senegal reached the Africa Cup of Nations final, which it lost 1-0 to Algeria on July 19, and he is scheduled to return next week.
"I don't think we can expect to play our best game of the season on Sunday – that's how it is with the preseason we've had and the long season after it," said Klopp, whose side finished runner-up to City and won the Champions League.
"But it's not about that, it's about winning a football game, and we'll try to do that."
Klopp, who was irritated about the game with City being called a friendly, added, "I would like to try to win it – it's a final.
"This is the first time I realized that nobody sees it like that, to be honest. 'A curtain-raiser', it's unbelievable."