Javi Garcia and Benat joined Alonso as the three cuts made to Del Bosque's provisional squad of 26, all three of them midfielders as Soldado survived.
News of Alonso's groin problem emerged last week, and Del Bosque was therefore expected to retain either Benat or Garcia to give himself more options in the middle of the park.
Spain will travel to the United States for friendlies against Haiti in Miami on Saturday and the Republic of Ireland in New York a week on Tuesday before heading south to Brazil.
Spain will play in Group B at the Confederations Cup along with Tahiti, Uruguay and Nigeria.
Spain squad: Casillas (Real Madrid), Reina (Liverpool), Valdes (Barcelona); Alba (Barcelona), Albiol (Real Madrid), Arbeloa (Real Madrid), Azpilicueta (Chelsea), Monreal (Arsenal), Pique (Barcelona), Ramos (Real Madrid); Busquets (Barcelona), Cazorla (Arsenal), Fabregas (Barcelona), Iniesta (Barcelona), Martinez (Bayern Munich), Mata (Chelsea), Navas (Sevilla), Pedro (Barcelona), Silva (Manchester City) Xavi (Barcelona); Torres (Chelsea), Soldado (Valencia), Villa (Barcelona).