Niemann's surgery to repair his labrum and rotator cuff will be performed Wednesday in the Dallas-Fort Worth area by Texas Rangers team doctor Keith Meister, who gave the pitcher a second opinion Monday.
The 30-year-old Niemann competed for the fifth starter's spot in spring training but went to the bullpen after Roberto Hernandez won the job. Niemann said he couldn't get himself to the point where he was ready to pitch every day if needed.
Niemann went 2-3 with a 3.08 ERA in eight starts in 2012. He broke his right leg last May and returned Sept. 1, only to leave that game with shoulder problems. He hadn't pitched this season when he was placed on the disabled list Friday with right shoulder soreness.
"During the spring we had time in between starts to manipulate and save it all, so to speak," said Niemann, who estimated his recovery time at nine to 12 months. "It went downhill so fast (in the bullpen), it was really the first time we experienced any kind of backtracking at all during this whole time."
Niemann said the extent of the damage won't be known until after surgery, but the decision was easy when he had trouble lifting his arm over his head and taking off shirts while preparing for bullpen duty.
Niemann was coming off one of his best starts against the New York Yankees last year when he was injured by a line drive against Toronto.
"It's really frustrating because that was the best I'd ever been as a pitcher in my career at any level," said Niemann, who is 40-26 with a 4.08 ERA in 97 career appearances, including 92 starts. "On the good side, that's what we have in our head to get back to."