Niemann, who will be activated on Saturday when rosters can be expanded, will pitch that day, pushing David Price back until Sunday.
That also means that James Shields will start the first game of the Yankees series at Tropicana Field on Monday afternoon.
Niemann, who has been on the disabled list since May 15 after fracturing his right fibula, said he is "ready to go" and that he "felt great" during his bullpen on Tuesday.
"The ball is coming out great, and I feel really good about how I'm throwing the ball," Niemann said.
Pitching coach Jim Hickey said that Niemann's stuff looked better on Tuesday than it did before he was injured. Hickey explained the rationale of having a sixth starter for an outing or two even though the five pitchers in the current rotation have been performing well.
Niemann's return is "going to come at a good time," Hickey said. "A couple of the younger guys, [Alex] Cobb and [Matt] Moore, really haven't pitched continuously like this in their careers, so maybe it's a chance to slide [Niemann] in there to see where he's at, see where we're at and give us just a little bit of extended time between starts for those guys."
Niemann just wants to help the team any way he can.
The rotation has "been doing great things without me, and I want to come in and just continue the trend," he said. "And I feel like we can do that and just make the team better. And that's what I'm trying to do -- make the team better and contribute."
Niemann was injured in Toronto when he took a line drive by Adam Lind off the lower part of his right leg. He smiled when asked if he could appreciate the irony of making his comeback in Toronto.
"When that was first run by me -- that that was a possibility -- I was like, 'Are you guys serious? I've got to go face the dragon the first time out? I've got to see the demon?'" he said. "But that's baseball. That's how it works. Might as well be the first one."