Niemann experienced slight inflammation in his right rotator cuff in September.
"I mean it is, but we're just trying to deal with what we've got to deal with now," Niemann said when asked if this injury felt similar. "Just go from there. Try to move forward."
Niemann said his shoulder began to bother him the second day of the season while trying to adjust to staying loose and ready as a member of the bullpen.
"Just got to the point where it would be tough for me to go out there and compete at the level I need to compete at," Niemann said. "Once you're at that point, you're putting yourself in a bad spot and you're putting your team in a bad spot, so [I] had to say something."
Niemann had not yet seen a doctor when he talked to reporters prior to Friday night's contest vs. the Indians at Tropicana Field.
"I'm not sure what the next course of action is going to be yet," Niemann said. "There's pain and just overall achiness. Not where you want to be when you're a pitcher. There's no way to hide that."
Niemann finally reached a tipping point while sitting in the bullpen during Thursday's Rays-Orioles contest at Tropicana Field.
"When you're sitting down there hoping the phone rings and it's not for you because you don't know if you can go throw a baseball, that's kind of as clear as it got for me," Niemann said. "… It's unfortunate, and some other words I can't say right now. But it is what it is and I we all have to deal with that at some point during our career. And here we are again, so figure it out."
Rays manager Joe Maddon called the news "disappointing" for the Rays and for Niemann.
"I really envisioned him doing some good things for us this year. He still can," Maddon said. "Let's see what the doctor has to say and we'll move forward from there."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.