The sick bay at the Metropolitan Opera is reaching a crescendo.

A knee injury has forced top Wagnerian soprano Christine Brewer to withdraw from her performances as Bruennhilde in the Met's "Ring" cycle, the company announced Thursday. In another major casting change, the Met also said that tenor Rolando Villazon will not sing the role of Nemorino in Donizetti's "L'Elisir d'Amore" on Tuesday and on April 4 _ a much-anticipated appearance by a tenor whose six-month absence from the stage last year led to speculation that he was suffering from vocal troubles.

The Mexican tenor, one of opera's most bankable stars, has laryngitis and is to be replaced by Massimo Giordano, the Met said.

Brewer, an American singer considered one of the world's great Bruennhildes, will miss the final three operas of Wagner's four-work cycle _ "Die Walkuere," "Siegfried," and "Goetterdaemmerung."

She has sung the part of Bruennhilde in concert, but this would have been the first complete "Ring" cycle of the 53-year-old singer's career.

Instead, the Swedish soprano Irene Theorin is to appear as Bruennhilde in the April 6 performance of "Die Walkuere," which will mark her Met debut, as well as in the same work on April 11 and the April 18 performance of "Siegfried."

Another Swedish soprano, Katarina Dalayman, is to sing Bruennhilde for "Walkuere" on April 25 and "Goetterdaemmerung" on April 28 and May 2.

Linda Watson, already scheduled to sing Bruennhilde in the third complete "Ring" cycle in May, is also to appear in "Siegfried" on April 30.

Brewer's manager, Alison Pybus, said the soprano injured her knee several months ago, when she slipped on ice while walking down the driveway of her home in St. Louis and landed on her knee.

"At first, she didn't think a lot of it," Pybus said. "It bothered her, but she started to rehearse at the Met."

After a couple of days on the Met's raked stage, however, "it really was causing her tremendous pain," the soprano's manager said.

An MRI and X-rays determined that Brewer will require knee surgery, at a date yet to be scheduled, Pybus said.

Brewer hopes to return to the stage on May 15, when she is to appear at Carnegie Hall as soloist for Mahler's Symphony No. 8. "This doesn't require major movement, since it's a concert," Pybus added.

The Met's "Ring" performances, conducted by music director James Levine, are the last time the company is presenting Otto Schenk's productions of the Wagner tetralogy that premiered with a performance of "Die Walkuere" on opening night of the 1986-87 season.

As Bruennhilde, Watson has sung at the Bayreuth Festival for the past three seasons, Theorin at her home theater, Copenhagen's Royal Danish Opera, and Dalayman in Stockholm and Aix-en-Provence.

Giordano, Villazon's replacement in "Elisir," sang the role of Alfredo in Verdi's "La Traviata" and Rodolfo in Puccini's "La Boheme" at the Met earlier this season. He made his company debut in 2006 as Des Grieux in Massenet's "Manon."

Villazon, who received good reviews for his return to the stage in January in Massenet's "Werther" at the Vienna State Opera, has since canceled several major performances in Europe and sounded in bad vocal shape weeks later in the Met's "Lucia di Lammermoor." He then bowed out of a worldwide HD broadcast of the Donizetti work because of illness.

The tenor plans to return for performances of "Elisir" on April 8, 11, 15, and 18.

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