Yet he managed it, winning enthusiastic reviews from critics who could barely believe the circumstances behind his performance.
The reviews cemented his reputation as one of the coming names in opera - as well as the tenor for a crisis.
He was due to arrive in Britain yesterday to begin rehearsing for Glyndebourne where he will open the festival next month in the title role of another Donizetti opera, Poliuto.
Fabiano, from New Jersey, described yesterday how he had been photocopying documents when his manager called last Thursday to see if he would stand in at the Met, one of opera's greatest venues.
"I asked when. And he said now, today," said Fabiano. "I said, 'Okay, I need a few minutes.'"
He ran back to his house to check he could remember the score. Part of the job was done. With Poliuto already in his head for Glyndebourne, Fabiano said he was already in "Donizetti land", giving him the right voice for the part.