The Giants initially thought they would have to face Robert Griffin III without Phillips, who was listed as doubtful with an injured right knee, and Pierre-Paul, listed as questionable with a back injury.
Fletcher hurt his left ankle against the Philadelphia Eagles on Nov. 18. Williams, already dealing with ankle and shoulder injuries, picked up a left thigh bruise against the Dallas Cowboys when kneed by a teammate.
Giants receiver Domenik Hixon is inactive for a second game in a row with an ankle injury. But safety Kenny Phillips (right knee), defensive end Jason Pierre-Paul (back) and offensive tackle David Diehl (shoulder) are active. Sean Locklear is expected to start for Diehl.
The Associated Press contributed to this report.