Peavy got the ball Tuesday night, which means he can also pitch the finale of the three-game series against the Tigers on Sunday.
Doubront will pitch Wednesday and then again on May 20 against the Blue Jays.
"We had the off-days to work with," said Farrell. "We just wanted another right-hander lined up against Detroit given the way their lineup stacks up, what they're doing against left-handed starting, and more than anything, just the flexibility of the schedule that gives us a chance to match up a little bit better."
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. This story was not subject to the approval of Major League Baseball or its clubs.