The Athletics optioned Bassitt to Triple-A Nashville on Sunday, Jane Lee of MLB.com reports.
Bassitt's demotion clears a spot on the 25-man roster for Frankie Montas, who was recalled from Triple-A in a corresponding move to start Sunday's game against the Diamondbacks. The 29-year-old right-hander was up with the Athletics for just one day and went unused out of the bullpen in Saturday's 3-0 victory.