Steffen is set to start at Wembley on Saturday ahead of usual No.1 Ederson.
And after playing every round of this season's FA Cup, Guardiola says his players would not support a decision to leave out the former Columbus Crew goalkeeper, even in such a high-profile fixture.
"He'll start, always I've done it like this," Guardiola told a news conference on Friday. "Maybe we are going to win or we are going to lose, but if Zack doesn't play I'm going to lose the team and it is better to lose a game than lose the team.
"He played really well in the FA Cup, he's a keeper from the national team USA, and when he plays, he plays at a good level. He is a lovely guy, training good and he deserves it. I'm more than delighted to give him the chance."
Steffen has made 10 appearances for City after returning from a year long loan at Fortuna Dusseldorf last summer.
He has deputised for Ederson against Chelsea already this season in a 3-1 win at Stamford Bridge in January, but ahead of the semifinal Guardiola has urged the 26-year-old not to try to copy the Brazilian.
"He has the personality, he has composure but no one in the world is like Eddie with the quality of his feet -- in the short, in the long, nobody," said Guardiola.
"I don't want Zack to do what Eddie does. He has to do what he can, try to save the balls, concede as few chances as possible and after that do a good game. We trust him a lot and that's why he's here."