His roster spot will be taken by outfielder Jai Miller, who is being called up from Triple-A Omaha. A right-handed batter, Miller has 18 home runs, 57 RBIs and a .252 average. He was claimed off waivers from Oakland on April 26.
Bannister had been assigned to the bullpen while trying to work out some mechanics so he could return to the rotation. He last pitched on Aug. 2 in a start. His record in 22 starts is 7-11 with a 5.95 ERA.
His assignment to the disabled list was backdated to Aug. 8. He becomes the third starting pitcher on the disabled list, joining Gil Meche and Luke Hochevar. Reliever Robinson Tejeda also is injured.
"He has some weakness in his shoulder due to the tendinitis, so we backed him off a start to see if we could build him back up," Royals manager Ned Yost said.
"He's been working real hard on all of the exercises that he does and he's actually starting to feel a little bit better," Yost added. "He threw a side today and he felt pretty good. We'll throw him a side again in a couple of days and evaluate then and decide from there."
Yost said that, if the side sessions go well, Bannister could be sent to the Minors for a couple of rehab stats and be ready to return in September when the rosters are expanded.
The Royals had been operating with 13 pitchers, so bringing in Miller gives them an extra position player on the bench.
"The guy's got a lot of pop and can play the outfield," Yost said.
Miller, 25, was drafted by the Florida Marlins in the fourth round of the 2003 First-Year Player Draft. His only Major League game came with them in '08 and resulted in one hitless at-bat.