Davis has not been scratched from his next scheduled start, Tuesday at Detroit, but the Rays held right-hander Jeremy Hellickson out of Sunday's game at Triple-A Durham in case Davis can't go against the Tigers. Hellickson has made one start for the Rays this season, beating the Minnesota last Monday.
Niemann, 10-3 with a 3.12 ERA in 22 starts, 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.
The Rays used just five starting pitchers in their first 104 games, but Sonnanstine is the seventh pitcher to start for Tampa Bay in seven games since.