Ederson was left on the bench as Stefan Ortega played at Real Madrid in the midweek 3-3 Champions League semifinal first-leg draw.
Asked who was his No1, Guardiola insisted: "No, (Ederson) is the first choice.
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"He was ready to play against Madrid and (will be) tomorrow, and the next game. We make the decision for many reasons."
On facing Luton tomorrow, he added: "We have to (focus), the Premier league so important.
"It's 11 months we've been fighting for the title, we know what position we are in."