Buchholz, who has been dealing with right shoulder fatigue, gets Game 4.
That means that Peavy would likely pitch Game 7 if the series goes the distance. Peavy had been Boston's Game 4 starter in the American League Division Series and Championship Series.
"After he got through his work yesterday, I felt like the overriding thing was an additional day of rest for him," Red Sox manager John Farrell said of Buchholz. "He's going to go Sunday."
During an in-game interview with FOX, Peavy said pitching in the Fall Classic was a "dream come true."
"I started playing this game as a little boy, and I've played for 12 years professionally at the highest level and never had this opportunity," said Peavy. "It's something I'll cherish Saturday night, but it's about business and finding a way to win."
In Game 4 of the American League Championship Series, Peavy allowed the Tigers seven runs on five hits over three-plus innings.
"I had trouble just making some adjustments, but we got to work over the last few days, and I feel comfortable going into Saturday night," said Peavy. "No reason I shouldn't execute pitches and be ready to go find a way to win."
If, for some reason, Buchholz doesn't respond and needs to be scratched, the Red Sox have two potential replacements in Felix Doubront and Ryan Dempster, who were in the rotation for most of the season.
However, Farrell fully expects Buchholz will be a go.
"It depends on who's used the next two nights," Farrell said. "We know that we've got multiple guys down there who can go multiple innings. Dempster being one, Doubront another. If we have to piece it together, we'll adjust accordingly. To say that we're going to have a guy standing there to warm up alongside of Clay, that won't happen."
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. Jason Mastrodonato is a reporter for MLB.com. Follow him on Twitter @jmastrodonato. This story was not subject to the approval of Major League Baseball or its clubs.