NEW YORK — Last-minute cast changes at the opera house are always full of drama, but the one for Saturday afternoon’s performance of Puccini’s “La Bohème” at the Metropolitan Opera promises to go down in the annals of day-of-the-performance substitutions.

Soprano Kristine Opolais, who had just sung the title role of Puccini’s “Madama Butterfly” for the first time at the Met on Friday night, was awakened at 7:30 a.m. Saturday with a phone call from Peter Gelb, the Met’s general manager, who made a bold request: Could she possibly sing the role of Mimi that afternoon in the matinee of “La Bohème,” to replace an ailing Anita Hartig?

“It was crazy,’’ said Opolais, who added that she had not fallen asleep until around 5 a.m. after all the excitement of her “Butterfly” performance. “I said, ‘No, no, no — it’s impossible. I would love to do that, but it’s impossible.’ But then something happened, and after five minutes I said — ‘Why not?’ ”

The stakes are always high at the Met, with its 3,800 seats, but Opolais’ last-minute switch into a role she had never rehearsed on stage would play out before a much wider audience. The performance, broadcast on radio and transmitted live to movie theaters around the world as part of the Met’s “Live in HD” series, was expected to reach more than 300,000 people.

Unlimited Digital Access. $1 for 4 weeks.

Opolais, who said that she would normally be resting her voice after a night singing “Butterfly,” found herself singing her second Puccini opera, and playing out her second Puccini death scene, on the stage of the Met within the space of 18 hours. While she had sung the role of Mimi at the Vienna State Opera and elsewhere, she had never performed it at the Met.

Opolais said Saturday after the performance that her streak would have to end. The Met’s Saturday evening performance was of Giordano’s “Andrea Chénier.”

“I was joking to Peter Gelb that in case something happens, I don’t know this role,” Opolais said. “Not yet.”