Hamburger was 7-4 with a 3.88 ERA in 27 relief appearances and four starts for Round Rock. With Scott Feldman making a spot start for the Rangers on Tuesday, they wanted to make sure they had a long reliever behind him.
"We needed some length, and he can give it to us," manager Ron Washington said. "He can get outs, he can spin the ball and as the season has progressed, his velocity has jumped. He can throw strikes. For a long guy, that's what we need, and he's the one that has been getting it done. We saw him in Spring Training and he wasn't afraid."
Hamburger was not on the 40-man roster. To make room, the Rangers released pitcher Ryan Tucker. Hamburger was at the Ballpark early on Tuesday for his first day in a Major League uniform.
"This means everything. ... It's a life goal just to walk into this place, a dream come true," Hamburger said. "I have never been in this stadium before. When I walked down the walkway to the field, I was overwhelmed. I'm more excited than anything and ready to pitch."
O'Day was placed on the disabled list with inflammation in his right shoulder. He has already missed two months early in the season because of surgery to repair a torn labrum in his left hip.
"It got sore when I was in Triple-A last Sunday," O'Day said. "I threw two innings, 40 pitches and it was sore the next couple days. I came up here on Wednesday and threw two more innings. I would guess that's when that happened. Yeah, it is [frustrating]. What more is there to say? Yeah, I don't want to be on the DL, two days before September, or at any time, for that matter. Yeah, it's disappointing."