Complex sentence: Verdi turned down an offer from Naples, but became concerned about the availability of his preferred Azucena, Rita Gabussi-De Bassini, but she was not on the Naples roster but was interested in the possibility of Rome.
Split into simpler sentences: Verdi turned down an offer from Naples, but became concerned about the availability of his preferred Azucena, Rita Gabussi-De Bassini. She turned out not to be on the Naples roster, but expressed an interested in the possibility of Rome.