Manningham suffered a strained calf muscle in Thursday's preseason finale, while Holliday was battling a hamstring injury throughout camp, Graziano reports.
Both players entered camp expecting to be key contributors to the team, but neither appeared likely to make the team at any point in August, according to Graziano.
Manningham, whom the Giants re-acquired after two seasons with the 49ers, has 2,849 receiving yards and 19 touchdowns in six NFL seasons. He played only six games last season for San Francisco and left Thursday's game against the Patriots after four plays.
Holliday was brought in to help the Giants' anemic return game, but that responsibility could now fall to receiver Rueben Randle. Holliday, a 5-foot-5 spark plug, had two punt return touchdowns and two kick return touchdowns in two seasons with the Broncos.
- Chris Mascaro