Soprano Christine Brewer, citing a knee injury that keeps her from moving on stage, has pulled out of next month’s run of Wagner’s Ring cycle at the Metropolitan Opera. Her appearances as Brünnhilde, Brewer’s first in a complete, fully staged Ring, were among the most anticipated events of the season. Meanwhile, vocally troubled tenor Rolando Villazón has withdrawn from at least the first two performance of Donizetti’s L’Elisir d’Amore, beginning next week.