"Quite frankly, our feeling was Ambriz needed a break," Astros general manager Jeff Luhnow said. "He's a good Major League pitcher who's not had a lot of success the last couple of weeks. We feel like there's probably a few things he needs to work on down there to get back in a groove so he can help us later this year."
Oberholtzer was a starter for the Redhawks, posting a 4.60 ERA and 5-6 record in 15 starts. It is his second time with the Astros this year. He gave up two solo homers to the Indians on April 21 in a relief appearance.
Manager Bo Porter and Luhnow said Oberholtzer would most likely see work as a long reliever, possibly as soon as Tuesday night's game against Tampa Bay.
"We wanted to get somebody down in the 'pen with starting ability and could give us length now that [Jose] Cisnero has been pitching more on the back end," Porter said. "The move just made sense given the available innings we had down in the bullpen."
Ambriz has appeared in 39 games for the Astros this year, tallying a 5.40 ERA and two saves as a late-inning mainstay.
Chris Abshire is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.