Left-hander Nate Robertson will return from injury rehabilitation to take Galarraga's rotation spot, general manager Dave Dombrowski said.
Galarraga, 6-10 with a 5.05 ERA, was sore after his last start in Oakland. Although the inflammation requires Galarraga to miss his next start, Dombrowski said he'll be back for the stretch run.
"We've got to take care of him," manager Jim Leyland said of Galarraga. "I don't want that on my resume. ... When you take care as much as you can during the regular season, you tend to have them fresher for the last month, if you happen to get that far."
Robertson has been on the disabled list since June 27 with a left elbow problem. He looked sharp in a start for the Mud Hens on Tuesday night, throwing 6 2/3 scoreless innings.
"Our reports on Nate were very good," Dombrowski said. "His slider and changeup were good. He feels healthy."
With a day off Thursday after a six-game California road trip, Porcello is the only Detroit starter who won't get an extra day of rest.
The moves won't affect either pitcher's postseason eligibility, Dombrowski said.