Buchanan started for the Astros against the Rays on Saturday in place of Peacock, who was recovering from food poisoning. In 4 1/3 innings, Buchanan gave up five earned runs and eight hits in an 8-0 loss in his Major League debut.
He'll move to the bullpen but was not available to throw Tuesday night, considering the close proximity to his last start.
"The decision was more predicated on allowing Peacock to do everything he needed to do to start on Friday," Porter said. "That's why we made the move today."
Mike Vernon is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.