Greg Bird is starting at first base, with Aaron Hicks in center field and Jacoby Ellsbury at designated hitter. Holliday has struggled in the second half, and Girardi said that the Yankees essentially saw three players for two spots, with Headley becoming the odd man out.
"Our thought was that runs are sometimes very tough to come by in playoff games, and we thought we'd put our fastest and most athletic team out there and see if we can do some things," Girardi said.
Headley accepted his changing role well following the July acquisition of third baseman Todd Frazier from the White Sox, taking on the challenge of playing first base while finishing the year hitting .273/.352/.406 with 12 homers and 61 RBIs.
"It's tough. It's really tough," Girardi said. "I'm sitting there the last week knowing I'm going to disappoint someone. It's really difficult. His at-bat maybe later on in the game could be the biggest at-bat in the game. That's what we told him."
Girardi said Headley's switch-hitting ability makes the bench deeper, and that Holliday could be the first choice off the bench to pinch-hit for Ellsbury if the Twins bring in a left-hander.
"It's certainly different when you're used to playing every single second of every game," Headley said on Tuesday. "You almost have to check out a little bit; not that you're still not paying attention, but the intensity is a lot more difficult to keep that up for 3 1/2 hours. For me, I try to relax just a little bit, maybe take a step back and not stay as focused as I would be."
Holliday had a productive first half, but he batted just .179/.225/.300 after the All-Star break, finishing his season hitting .231/.316/.432 with 19 homers and 64 RBIs in 105 games.
"His at-bats have not been as consistent because of some of the trades that we've made and increasing the amount of players that had the ability to play every day, so it's kind of cost him at-bats," Girardi said. "If it was a left-hander, [Holliday] would probably be out there tonight."
Bryan Hoch has covered the Yankees for MLB.com since 2007. Follow him on Twitter @bryanhoch and on Facebook. This story was not subject to the approval of Major League Baseball or its clubs.