The 29-year-old, who fractured his right fibula on May 14, has been able to play catch since Tuesday after getting off the scooter Monday.
"It's progressing the right way, and it's good to be standing up again," Niemann said.
There is still no timetable for his return, and the biggest challenge is getting the full range of motion back. He added he isn't sure what the next step is or when it will be.
To maintain as much arm strength as he could, Niemann said he was able to throw a ball into a trampoline when he was in the cast, playing catch with it.
"I think we did a pretty good job, because I'm throwing right now and the arm feels good, it feels strong," Niemann said. "It's going to be one of those things the sooner we are there, the faster we're going to be able to come back."