The 6-foot-9 right-hander has been on the 15-day disabled list since suffering a lower back injury on May 4. Manager Joe Maddon said Niemann could return to the Rays' starting rotation to pitch on Monday against Milwaukee, meaning Alex Cobb will make one more start on Saturday.
Niemann turned in 5 1/3 scoreless innings for the Bulls on Tuesday, striking out four batters and surrendering just three hits. He threw 92 pitches.
Before he went down with the injury, Niemann was 1-4 with a 5.74 ERA.