Guardiola said: "Take a look at our games and you realise. It is simple. He is so important for the quality he gives us. But if you have a player that doesn't want to play, he won't play.
"I need to rest the centre-halves also but in (England's) friendly games, (John Stones and Kyle Walker) got injured so they get cannot rest. We are in big, big trouble.
"I had the feeling we were tired in the last games. We will decide tomorrow what we have to do."
Guardiola was also asked about Erling Haaland and what he must do to become a Ballon d'Or contender.
He said: "He needs to play more minutes, to learn from what you have to do.
"But the target is not the Ballon d'Or, the target is to win trophies and he did. Would we have won five trophies last year without him? Not a chance.
"It's not about Erling or any other player. Until you retire, you can (always) be better."