The reason for the switch is that Buchholz, who missed three months earlier this season with a right bursa sac strain, is dealing with shoulder fatigue.
"It's just a little dead [arm] at certain points," said Buchholz. "It's not a pinpoint straight point where you say, 'That's where it is.'"
The fatigue perhaps explains why Buchholz hasn't looked like himself in any of his three postseason starts, posting a 5.40 ERA and giving up a .284 opponents' batting average. Buchholz has a no-decision in all three of his starts.
Before Game 1 of the World Series, Farrell was asked if Buchholz had a health issue.
"Not to the point of keeping him out of starting," said Farrell.
The difference between Buchholz pitching Game 3 or 4 is considerable, considering the following: If he had started Game 3, he could have come back for Game 7.
By pitching Game 4, it means he's likely to start just once in the World Series.
"That's being factored in," said Farrell. "I mean, I have to stay conscious of that, given the last two starts when he's hit the wall, it's happened pretty quick. All that is being factored in."
Buchholz has started fairly strong in all three postseason starts, but by the middle innings, he's completely lost his rhythm.
The third time through the batting order, Buchholz has been hit at a .529 clip. From pitches 61-75, batters are hitting .264 against him. From pitches 76-90, he's given up six hits in nine at-bats.
Peavy turned in a strong performance against the Rays in Game 4 of the Division Series, giving up one run over 5 2/3 innings. But he struggled mightily against the Tigers in Game 4 of the ALCS, allowing five hits and seven runs over three-plus innings.
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.