The club recalled right-hander Wilton Lopez from Round Rock and purchased the contract of left-hander Gustavo Chacin, putting the 40-man roster at its limit. Chacin will pitch as the lefty out of the bullpen in place of Byrdak.
Astros general manager Ed Wade said Byrdak's hamstring had been bothering him for several weeks.
"[Head athletic trainer] Nate Lucero called me after he came out of the game tonight and said Tim said he had trouble finishing his pitches, and Nate went back and looked at video of last year compared to this year and it was apparent to him he was really having trouble coming down on his landing leg," Wade said. "It seemed apparent he was trying to compensate for the hamstring."
Wade talked to Byrdak (1-0, 5.23 ERA in 13 games) after the game and he confirmed that was the case.
"Before the hamstring blows or he hurts his arm, we put him on the DL, and hopefully, it resolves itself in 15 days," Wade said.
Chacin was 1-1 with a 3.65 ERA in five starts for the Express. He's allowed 24 hits and six walks while striking out 14 batters in 24 2/3 innings. He was a non-roster invitee to Spring Training.
Wade said left-hander Wesley Wright was also given consideration to be called up , but he threw six innings Monday.
"I talked to [Round Rock manager] Marc Bombard and he said Chacin could handle the role in the bullpen, and [pitching coach Brad Arnsberg] is familiar with him from Spring Training and their days together in Toronto," Wade said.
Gervacio, who threw one-third of an inning Monday and gave up a two-run single, is 0-1 with a 12.27 ERA in 3 2/3 innings over six appearances. Lopez was 2-1 with a 5.40 ERA in three relief appearances at Round Rock and was 1-0 with a 4.50 ERA and five strikeouts in four relief appearances for the Astros earlier this year.
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.