Ranaudo, 24, is scheduled to start at Cincinnati on Wednesday. Right-hander Brandon Workman originally was scheduled to make that start, but he is being pushed back after pitching in relief during Boston's 5-4, 19-inning loss against the Angels in California on Saturday.
Ranaudo fills the roster spot opened up when left-hander Edwin Escobar was optioned to Pawtucket on Monday.
This is Ranaudo's second stint of the month with Boston. He got the win in a start against the Yankees on August 1, allowing four hits and two runs with four walks and two strikeouts in his major-league debut.