The midfielder has complained about fatigue this week.
Ahead of facing Luton, Guardiola said: "If a player doesn't want to play then he's not going to play, simple.
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"Take a look at our games and then you realise. He's so important for the quality that he gives.
"I have the feeling that the games when he was tired like against Crystal Palace and Madrid, he was better in the second half. He runs more and was more precise in the second half than the first.
"More than the physicality, it is a case of spending mental energy. Playing every three days, three days. Of course he's tired, playing a lot of minutes."