Worley's next turn would have come up Wednesday, in the second game of a two-game Interleague series against the Twins in PNC Park.
Monday's bye allows Jeff Locke to move into that slot behind Francisco Liriano, who will pitch the opener against Minnesota. Another off-day after that two-game series will let the rotation reset. By the time a fifth starter is needed, Morton will be ready to merge in.
Morton, on the DL all season recovering from late-September labrum surgery, is scheduled for his third and final rehab start on Tuesday in Indianapolis.
Neither Worley nor Locke have pitched very effectively the past couple of weeks. But Worley looked flat on Thursday against his former Philadelphia team, and the Pirates may have had possible concerns about a similar dynamic hurting him against the Twins, his other ex-team.
Locke's good track record in Interleague play may have also factored into the decision: He is 4-1 against American League teams, with a sharp 2.56 ERA in six starts.
Tom Singer is a reporter for MLB.com and writes an MLBlog Change for a Nickel. He can also be found on Twitter @Tom_Singer and on his podcast. This story was not subject to the approval of Major League Baseball or its clubs.