"He feels he can't compete at this level without it,'' said head athletic trainer Ron Porterfield. "He feels like right now it's bothering him too much that he just doesn't have his confidence that he can do what he needs to do.''
Niemann, 30, was examined by Rangers team doctor Keith Meister and was given a choice between surgery or an injection plus rehab. He has damage to his labrum and rotator cuff. He will have one more exam on Tuesday before having the surgery.
Niemann had his first shoulder surgery in October 2005, which corrected an impingement among other things. He missed about a month last season with a shoulder strain and the joint has given him some nagging trouble in the past.
The fourth overall pick in the 2004 draft has gone 40-26 with a 4.08 ERA in parts of five big league seasons. He lost a spring training competition for the team's final rotation spot to right-hander Roberto Hernandez.