Davis and Niemann were sent back to Florida from Toronto on Sunday to be examined by team doctors.
Niemann, 10-3 with a 3.12 ERA in 22 starts, missed his scheduled start Sunday. He felt pain after beating Minnesota on Tuesday and was still sore after playing catch Saturday.
Davis, 9-9 with a 4.45 ERA in 21 starts, felt sore two days after his last outing, an Aug. 5 loss to Minnesota.
Hellickson won his MLB debut on Aug. 2 against the Minnesota Twins. He struck out six and walked two in seven innings. He will start Tuesday in place of Davis.
Information from The Associated Press contributed to this report.