According to team spokesman Rick Vaughn, Niemann was expected to arrive Friday night, but the team has until Sunday to make an official roster move.
Niemann, 25, who is 1-1 with a 3.27 ERA in two starts for Triple-A Durham, was the Rays' top pick in the 2004 First-Year Player Draft. He has battled injuries throughout his career, but was healthy in 2007 and pitched well this spring, posting a 2-0 record with a 1.50 ERA in four games.
Losing Scott Kazmir and Matt Garza to early-season injuries has taxed the Rays' starting rotation. Now the depth of the organization's pitching will be put to the test with Niemann's arrival.
Despite Niemann's disappointment at getting sent down in the latter stages of Spring Training, the 6-foot-9, 260-pound right-hander said at the time that he felt better about his pitching than he has at any point in his professional career.
"I'm more in control, the way it's feeling, it's more natural," Niemann said. "I'm not searching for things. That's the biggest thing. I have a plan of attack and keys I can go back to and make adjustments, whereas last year, I was just throwing it, hoping for the best."
Niemann also made an impression on Maddon.
"With 6-foot-9 throwing downhill, it is very attractive," said Maddon on the day Niemann was cut. "It's an abnormal angle to hit from when a guy's that tall throwing the ball down. You don't get to see that every day as a hitter. It's an exclusive kind of a way to throw a baseball."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.