"We feel we need to get it down to a five-man rotation and go with it," Girardi told Major League Baseball's website (www.mlb.com).
The decision could leave either A.J. Burnett or Phil Hughes in a reduced role.
Burnett has not won a game in seven starts since late June, going 0-3 with a hefty 6.0 earned run average (ERA). The right-hander (8-9 overall) gave up four runs in one inning on Tuesday in an eventual loss to the Los Angeles Angels.
Hughes, another right-hander, is 2-3 with a 4.55 ERA in six games, five of them starts, since returning from the disabled list in July. He won 18 games in 2010 but is 2-4 this season.
There is only one spot open in a five-man rotation because the previous odd-man out, Ivan Nova, apparently has pitched well enough recently to earn a starting spot with CC Sabathia, Bartolo Colon and Freddy Garcia.
Nova is unbeaten in three starts since coming back to the big leagues in late July.
He won a starting job in spring training and went 8-4 before being demoted to the minor leagues. He is 11-4 for the season.
(Reporting by Gene Cherry in Raleigh, North Carolina; Editing by Steve Ginsburg)