When Peter Gelb, the general manager of the Metropolitan Opera, walked onto the stage before the Saturday matinee of Wagner’s “Walküre” to announce that the tenor Jonas Kaufmann had withdrawn because of illness, he had an unusual twist to add.


The substitute for Mr. Kaufmann, the intended Siegmund, was the Dutch tenor Frank van Aken, making an unexpected house debut. Mr. van Aken is married to Eva-Maria Westbroek, who is singing Sieglinde in this second run of Robert Lepage’s Wagner “Ring” cycle.


“Wagner had more than he bargained for with a husband and wife singing Siegmund and Sieglinde,” Mr. Gelb said wryly, alluding to the incestuous, adulterous relationship of the characters, who are twins.


“We hope that as he finds his footing on stage, the machine will be kind to him, too,” Mr. Gelb added, referring to the mishap-prone, 45-ton mechanism of movable planks that constitutes the set for this production.


Even though Mr. van Aken never quite found his footing vocally, his was a heroic effort. He has sung the role at major houses including La Scala in Milan and the Frankfurt Opera. As he explained during a live radio interview after Act II, he had come to New York to hear his wife sing Sieglinde; this is his first visit to America.


Mr. van Aken, who last sang the role 16 months ago at La Scala, also as a late replacement, participated in a quick rehearsal on Friday afternoon with Fabio Luisi, who is conducting many performances in the cycle, and had only a few moments to try out his voice on the main stage, which was scheduled to be used for a lecture. He did not sleep all night, he added.


Ms. Westbroek, who has sung in this production before, urged her husband simply “to keep breathing.”


Given the stressful circumstances of Mr. van Aken’s debut and the substantial hiatus since he had last sung the role, it was not surprising that he sounded less than entirely confident, particularly during Act II, when he cracked a few notes and appeared to cough.


He sounded his best during the love duet in Act I, where there was certainly palpable chemistry between him and Ms. Westbroek. She sounded radiant throughout, singing with a rich, powerful voice that easily cut through the thick orchestral textures.


The Lepage set has been plagued by glitches, but there were no visible malfunctions on Saturday. The only alarming moment came from a runaway weapon: the spear of the god Wotan, sung by Bryn Terfel. In Act II, during Wotan’s argument with his wife, Fricka, the spear rolled down the stage, gathering speed until it seemed it might fly into the pit and impale a member of the orchestra. Fortunately, it came to a halt just a hairbreadth from the edge of the pit.