The team said Montas has a stress reaction in his right rib, and the procedure was done in Dallas. The 22-year-old pitcher will begin physical therapy next week at Camelback Ranch in Glendale, Arizona.
Montas was acquired from the White Sox with infielder Micah Johnson and outfielder Trayce Thompson in a three-team deal involving Todd Frazier. The Dodgers sent Brandon Dixon, Jose Peraza and Scott Schebler to the Reds as part of the deal.
Montas was 0-2 with a 4.80 ERA in seven appearances with the White Sox last season.