Guillen bowed out after going 0-for-2. Acting manager John Gibbons said the hamstring problem, which Guillen felt as he twisted in his swing, was probably related to the knee injury.
Bannister didn't know if he'd miss a start or not because of his fatigue, which has affected not only his shoulder but his back.
"I don't think it's anything serious, but I haven't been recovering the way I normally do, and after my last start I didn't recover at all," Bannister said. "I went out there and didn't have much at all today."
Bannister hasn't been on top his game since his last victory on Aug. 2 at Tampa Bay. He's made six starts since without posting another win.
"I haven't felt 100 percent since my start in Tampa," Bannister said. "I'm a professional and I take pride in taking the ball every fifth day even when I don't feel good. But it's progressively gotten worse over the last month, although I've been able to give this team innings."
Not this day, however. It was an inauspicious beginning for Bannister. It was four up, four on before he finally got an out.
Adam Kennedy led off the first inning with a walk, Rajai Davis unloaded a deep double to right-center field and both runners scored as Ryan Sweeney looped a single into center. Jack Cust followed by poking a single to left.
"It affected my ability to get the ball down in the strike zone consistently and I've given up over a run an inning for the last month ... it's not fair to my teammates or fair to the fans to go out there and have performances like that," Bannister said.
Three outs, including a sacrifice fly by Mark Ellis, followed, but Bannister's problems weren't restricted to a 3-0 deficit.
After Cliff Pennington popped out to open the A's second, Bannister was removed from the game because of the problem with his pitching arm.
Yasuhiko Yabuta, making his second appearance since being recalled from Triple-A Omaha, replaced Bannister. Before he could end the inning, the A's had shoved across five runs for an 8-0 lead.
Landon Powell applied the crusher, sending a grand slam soaring to right-center field, the first of the rookie's career. Yabuta gave up four other hits and a walk and, not surprisingly, that was the end of his afternoon.
The Royals got a bit rambunctious themselves in the third inning, scoring three runs against Trevor Cahill, a right-hander who had the dubious distinction of ranking third in the American League in losses (12) and second in home runs allowed (25).
Well, make that 26, because Mike Jacobs pounded a solo shot for his 17th homer in the third.
Left-hander Bruce Chen, who replaced Yabuta for the third inning, also had an unseemly introduction to this game. He began with two walks, gave up a single to load the bases, then plunked Davis with a pitch. Even so, Chen got Sweeney to hit into a double play (as another run scored) and fanned Cust, and the A's lead was a mere 10-3.
After that, though, Chen found himself and put together three scoreless innings to save some wear-and-tear on the beleaguered bullpen. The Royals had to use five relievers to nail down a 4-3 win on Tuesday night.
The hurry-up call didn't catch Chen off guard.
"I was ready, I was warmed up and everything," Chen said. "I felt good and kept pitching and got better command as I went along.
Roman Colon added two perfect innings to restrict the A's damage to the first three innings.
Along the way, David DeJesus helped with two sterling diving catches in left field -- one in foul ground and the other in left-center.
"He plays as good as any left fielder I've seen this year and in a big ballpark," Gibbons said. "Truthfully, who plays it better?"
The Royals' batters were dormant until the ninth, when Davis, the A's center fielder, somehow let Mitch Maier's fly ball drop for a double. Maier scored on Josh Anderson's single.
The Royals finished with a 2-6 record against the A's this season. The only Kansas City victories came in five games at the Coliseum.
"We were fortunate to get one win out of it," Gibbons said of this series. "It's always been a tough place to play."