Manning has started 207 consecutive regular-season games for the Giants.
After the Giants' 51-17 loss to the Los Angeles Rams on Sunday, McAdoo said that they need to look at the possibility of getting reps to players who have a chance to be part of their future.
"That includes everybody," McAdoo said when asked if the quarterbacks are among those in play.
Manning, who has 12 touchdown passes and nine turnovers this season, has started every game since midway through his rookie season in 2004. He's one shy of tying his brother, Peyton, for second on the all-time list.
"Well, they've got to do what they've got to do," Manning said. "I want to be out there, I want to keep playing with my teammates and keep working."
Webb is a third-round pick out of California who has been inactive for all eight games this season. Geno Smith came into Sunday's game to finish for Manning. Smith threw a pair of incompletions and took some hard hits in his only drive.
Manning is the Giants' highest-paid player at age 36. He went 20-of-36 for 220 yards with two touchdown passes and an interception on Sunday. He also lost a fumble, his third of the season.
The Giants suffered their third-worst home loss in the franchise's 92-year history on Sunday, and Manning again didn't play particularly well. He missed three potential long touchdowns in the first half, including a pass to Sterling Shepard deep down the middle of the field that likely would've gone for a 75-yard score in the second quarter. He later missed an open Tavarres King in the end zone on consecutive plays before halftime.
"Yeah, I've got to hit those," Manning said. "I've got to hit that, especially the one to Shep."
"It's disappointing," McAdoo said. "We had chances down the field. We got to make those throws and make those catches. It's pro football. We got to catch them on all opportunities like that. You don't get many in the game."
But the Giants' problems stretch beyond their quarterback. They're missing star wide receiver Odell Beckham Jr. and Brandon Marshall, who are out for the season with injuries. Their defense also has failed to live up to expectations, and their special teams have been troublesome.
The offense isn't operating smoothly, no matter who is at quarterback. The Giants are averaging just over 16 points a game, near the bottom of the league.
On Sunday, five different Giants combined to drop seven passes, according to ESPN Stats & Information. It was their most drops since Week 15 of the 2007 season.
Manning and the 1-7 Giants are closer to the No. 1 overall pick than playoff contention in this disastrous season. It's likely to be the fifth time in six years they miss the postseason.
Too many more weeks like this, and the Giants may be forced to turn to Webb. They would at least see what he brings to the table in order to determine whether they need to address the quarterback position high in next year's draft.
The Giants, who play the 49ers next Sunday in San Francisco, would have the third overall pick if the season ended after Week 9.