Rays manager Joe Maddon explained the move by saying there was a "three-pronged reason" for it.
"I wanted to break up Garza and [Edwin] Jackson -- very similar guys -- that's [the first reason]; [Second] we really wanted Shields to partake in that doubleheader game in Baltimore, based on his ability to go deep into the games," Maddon said. "And the last part was just to give Shields an extra day of rest based on the workload that he's had this year."
Neither pitcher seemed too concerned about the move.
"They told me to pitch on Friday, so I'm going to pitch on Friday," Garza said. "I just do what they tell me. ... It doesn't bother me none."
Shields insisted he wasn't anywhere near tired, but did not seem ruffled about changing.
"It's just a decision they're making right now," Shields said. "It is what it is, and I just have to pretty much do what they want right now, and what's best for the team."
The move also means Garza will now pitch against the Red Sox at Tropicana Field on Sept. 17, while Shields will miss the Red Sox series and pitch against the Twins on Sept. 18.
"Good thing for me," said Garza when asked about pitching against the Red Sox. "Bad thing for them."
Shields will now pitch the first game of the Sept. 23 doubleheader at Baltimore, which could help save the bullpen on a potentially long day.
"When Shieldsy goes, it's going to be seven [innings], possibly eight," Maddon said. "So being that it's the first game of the doubleheader, that was part of it. They're all interconnected."
Maddon opened the door for speculation that the nightcap of that doubleheader could mark the Major League debut of top prospect David Price, since an extra pitcher will be needed due to the fact the team has no more off-days after Thursday.
"We have discussed candidates [to pitch the second game]," Maddon said. "We have not decided yet. They have a lot of left-handed guys who are good, we'll just wait and see. We're talking about [Nick] Markakis and [Aubrey Huff]. We'll just wait and see."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.