Martinez was optioned to Triple-A Round Rock on Thursday, Stefan Stevenson of the Fort Worth Star-Telegram reports.
The team had to clear a spot on the roster for Dario Alvarez, who was acquired from the Braves on Wednesday, and Martinez was the casualty. Martinez was an obvious candidate for demotion, having given up five runs over four innings Tuesday and 20 runs total in his five starts (23.2 innings). Lucas Harrell, also brought in from Atlanta, is expected to take Martinez's spot in the starting rotation.