Cech endured a mixed performance against the champions, making a fine double-save to deny Riyad Mahrez and Aymeric Laporte before thwarting Sergio Aguero when one on one, but also failing to get close to Raheem Sterling's opener and looking unconvincing when trying to play out from the back.
Leno had widely been expected to be Arsenal's first-choice keeper this season following his £22m arrival from Bayer Leverkusen, but Emery was pleased with what he saw from veteran Cech.
"We want competence with the goalkeeper and the other players in the squad. The pre-season offered the same chances for all the goalkeepers," he told reporters.
"Petr Cech had a very good pre-season. Today I think he played well also and he has this experience to defend our goal.
"Leno is starting with us, working very well and also played well in pre-season. But he can wait and arrive in his moment."
Cech made more mistakes leading directly to goals than any other player in the Premier League last season.