City meet Forest on December 27, having defeated West Ham on Saturday.
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Guardiola said, "The moment they arrive after three days (off) I want to see how they come back. They can eat but I want to control them.
"Imagine one player and now he's perfect but he will arrive with three kilos more.
"He will be in Manchester, he will not travel to Nottingham Forest."
On Christmas, Guardiola says it's crucial his players spend time with their families this week.
"I've learned from England, since I arrived, as much days (where you can) have a day off, you give them," he added.
"The schedule is so tight and the players have to forget. The moment of the game they will be fresh in the legs."