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"It was an accident, but we need him to try because that's how we play our football," Guardiola told reporters after the game.
"He deserves to play and after that he had a good game," Guardiola said. "He made good saves and is an exceptional keeper."
"We use our keeper to move our opponents into different situations. I'm pretty sure Zack didn't want to do it," Guardiola said. "Sometimes the strikers miss in front of the keeper, other times the keeper makes a mistake."
Steffen was one of the seven changes Guardiola made from the team that started in their Champions League game in midweek, with the City boss opting to rest several of the team starters including first choice keeper Ederson.
City are top of the Premier League on 74 points, one point ahead of Liverpool, with seven matches left. They host Brighton & Hove Albion in the league on Wednesday.