The fear that Manolin is feeling is related to his concern about the possibility that "the Indians of Cleveland" might defeat the Yankees. Manolin is not actually asking about fear in terms of being afraid of something or someone.
Santiago reassures Manolin with the encouragement "Have faith in the Yankees my son. Think of the great DiMaggio." Manolin persists, stating that he is concerned about both the Detroit Tigers and the Cleveland Indians as being teams that might beat the Yankees. Santiago, of course, has complete confidence in the Yankees as a team and in Joe DiMaggio's supreme abilities. Santiago tells Manolin that he shouldn't be so doubtful, that he shouldn't let his fears for the Yankees get out of control, commenting "Be careful or you will fear even the Reds of Cincinnati and the White Sox of Chicago."