Pettitte was originally scheduled to start Wednesday's finale against the Blue Jays, but when the Yankees clinched a playoff spot with Tuesday's 6-1 win, his start was changed so that he could go into the playoffs closer to regular rest. Javier Vazquez started Wednesday in what could be a postseason audition.
Pettitte will be followed Saturday by A.J. Burnett, and manager Joe Girardi introduced Sunday's starter as "Mr. TBA."
This will be Pettitte's third start since coming back from being on the disabled list from July 19 until Sept. 18 with a groin injury.
Burnett gave up seven runs in 2 1/3 innings against Toronto in a 7-5 loss on Monday and is 10-15 with a 5.33 ERA, making Saturday's start important.
"We want him to pitch well," Girardi said. "We need him. A.J. has been a big part of our success last year, and we need him to get right. He struggled the other day with his command here. Saturday, I'm looking forward to a good start."
Girardi said Phil Hughes will get some work in one of the games in Boston. He said the situation in the standings, whether the division is on the line, could be a factor in determining Sunday's starter. Another factor could be if there are relievers who are part of the postseason plans who need work. Girardi does not want any of them to have too much time between appearances.