Astros general manager Jeff Luhnow announced Monday that Houston has optioned left-hander Fernando Abad to Triple-A Oklahoma City.
It clears a spot on the Astros' active roster for outfielder Justin Maxwell, who was claimed off waivers on Sunday from the New York Yankees.
The 26-year-old Abad has already made two appearances for the Astros this season, allowing an earned run while tossing 1 1/3 innings.
He made the Opening Day roster as one of the eight bullpen pitchers after posting a 2.08 ERA over nine relief outings this spring.
Maxwell will be in uniform for the Astros on Monday night for the start of a three-game series against the Braves at Minute Maid Park.
The 28-year-old Maxwell hit .260 (46-for-177) with 16 homers, 31 RBI and eight doubles for Triple-A Scranton/Wilkes-Barre last season.
Maxwell was selected by the Nationals in the fourth round of the 2005 First-Year Player Draft and has spent parts of three seasons (2007, 2009-10) at the big league level as a member of Washington.
Houston's 40-man roster now sits at 39.