When David Carr became an NFL quarterback in 2002, he had an unconventional wish for a first pick in the draft.

He wanted to be a backup.

Despite a glittering career at Fresno State, Carr, who was selected to be the cornerstone of the expansion Houston Texans, knew he had to learn how to be a professional, from how to handle the news media to how best to watch game film. In the classic NFL model he embraced, he would sit behind a veteran starter who would help groom him for the job.

It did not work out that way. The best available veteran quarterbacks often eschew expansion teams in favor of playing for contenders, and Carr, forced to start immediately as a rookie, says now that he believes it set his career back just as it was getting started.

Ten years later, after making almost 80 starts, Carr has the job he used to want. He is Eli Manning’s backup with the Giants, and he has not taken a single snap this season.

Carr’s career, which began too quickly and may have faded too soon because the Texans struggled to manage his position, has made him the ideal at the most dreaded and desperately important job in football: the one that late this season has been pushed into the spotlight like never before. The Texans’ former general manager, Charley Casserly, leans on the philosophy of his former boss Joe Gibbs: “The second most important person on the team is the backup quarterback.”

Nodding his head toward Manning’s locker a few feet away, Carr agreed.

“Around the team, a lot of guys get hurt, young guys go in, they pull guys off the street and they go in and start and it’s not magnified as much as when the quarterback goes down,” Carr said. “In our situation, Eli goes down, I can go in. I’m just trying to get our team a victory. I’m not trying to steal a guy’s position.”

The fraught dynamics and intense responsibilities associated with the quarterback position have made this season’s numerous quandaries especially compelling. From the Indianapolis Colts’ stumbling scramble to find an appropriate replacement for Peyton Manning to playoff contenders like the Texans and the Chicago Bears putting their late-season runs into the hands of backups — and, in the Texans’ case, third-teamers — the players who usually wear ball caps during games have instead worn the yoke of their teams’ hopes.

More than half the teams in the league have gone to their backups at some point this season because of injury or ineptitude. That list will grow this weekend when the Cleveland Browns and perhaps the Pittsburgh Steelers have to play backups, too, because of injuries to starters.

This weekend’s marquee game — the Denver Broncos against the New England Patriots — presents the extremes of a sometimes unwieldy affiliation. Tim Tebow, the Denver phenomenon, was supposed to be a backup this season, to learn behind a veteran like Kyle Orton. But Orton’s poor performance, accompanied by an extraordinary groundswell of public pressure, thrust Tebow into the starting role, where he is now 7-1, almost assuring that the Broncos will deal with another uncomfortable backup situation next season.

Orton has since been waived by the Broncos and picked up by the Chiefs, for whom he will start this weekend in place of Tyler Palko, the ineffective backup to the injured Matt Cassel. Tom Brady, a former backup who never relinquished the starting job after Drew Bledsoe was hurt in 2001, is so entrenched that even regular followers of the Patriots might struggle to name his backup. (It is Brian Hoyer, although the Patriots also drafted Ryan Mallett in the third round last spring.)

Carr’s situation, though, encapsulated what makes managing the backup quarterback such a vexing job. No position attracts such scrutiny. And no other is complicated by a toxic mix of bloated egos, crushing disappointments, undiscovered talent and overblown expectations.

Youth or age

General managers grapple with whether to groom a young player or bring in a veteran (the Colts), and with whether they can afford a top-flight backup if they have a top-flight starter (also the Colts). Coaches sometimes want to create competition for a less-than-secure starter (the Washington Redskins, among many), while hoping to avoid the friction that develops when having a quarterback-in-waiting behind an established veteran (see the San Francisco 49ers of the Joe Montana-Steve Young era).

Rare are Bob Griese and Earl Morrall, who coexisted seamlessly enough, and were both talented enough that when Griese was hurt early in the 1972 season, Morrall took over for most of the rest of the Miami Dolphins’ perfect run, until Griese was ready to take his job back in the Super Bowl. More common is the strained relations that occurred in Green Bay between Brett Favre and Aaron Rodgers.

“In theory, you’re looking at quarterback No. 33 in the league — how good is this guy going to be anyway?” said the former Colts coach Tony Dungy, who is now an analyst for NBC’s “Football Night in America.” “We actually thought the backup had to be a guy who could win you a game or two, if you needed it. Not ‘Can we win the Super Bowl with the backup?’ It’s not realistic. You can be successful but you’re not going to be a playoff team. It’s not like the days where you can have Bob Griese and Earl Morrall. It just doesn’t happen.”

That is largely because many teams fumble their chances to develop backup quarterbacks. Salary-cap constraints mean that some teams keep just two quarterbacks on the roster. Limited time in practice means the backups take few snaps. On some teams backups are discouraged from asking questions in quarterback meetings. Even in blowouts, coaches often do not let the backups play.

With the demise of NFL Europe, there is nothing approaching a developmental league. And Rich Gannon, the former quarterback — starter and backup — is surprised by how little coaching of fundamentals quarterbacks receive once they are in the NFL.

“Some teams, the guy coaching the quarterbacks has never played the position or coached the position before,” Gannon said. “This is the NFL. How do you do that?”

Highs and lows

No team’s struggles have been on more glaring display than those of the winless Colts, who hurriedly signed the veteran Kerry Collins during the preseason after the extent of Manning’s neck injury became known. With little time to prepare, Collins struggled. But when he got hurt and the job was turned over to Curtis Painter, the bottom fell out. Painter was drafted by the Colts in 2009, presumably to be groomed as Manning’s backup.

“It’s a crapshoot when you’re drafting guys who only play in the preseason,” Casserly said. “The Colts probably thought Painter was better than he ended up being.”

There are, though, success stories. New England won 11 games with Cassel, a seventh-round draft pick, after Brady was injured in the 2008 season opener, earning Cassel his own starting job in Kansas City. In Green Bay, the backup Matt Flynn, another seventh-rounder, is so well regarded after he filled in for Rodgers last year that he, too, could become a starter elsewhere. In Cincinnati, the backup Bruce Gradkowski has been instrumental in the rapid development of the rookie starter Andy Dalton because he knows the offensive coordinator Jay Gruden’s concepts from their shared time in Tampa Bay.

“There has not been animosity,” Bengals coach Marvin Lewis said. “He has helped move this thing forward. He’s like, ‘Hey, Andy, I’ve got this play at home, I’ll show it to you against cover-2, we ran this in Tampa.’ He’s been an unbelievable liaison.”

Best-case backup

But the most stunning success is back in Houston. A decade after Casserly was thwarted in finding the backups he wanted to help Carr, the Texans have won their first division title with their third-string quarterback.

When Matt Schaub was put on injured reserve, Matt Leinart, brought in for his veteran expertise, was supposed to take over. But when Leinart broke his collarbone in his first start, it pushed the rookie T.J. Yates — a fifth-round draft pick — into the job. The Texans, with one of the best offensive lines and defenses in the NFL, have won all three games in which Yates has played, giving them hope that their playoff dreams did not go down with Schaub and Leinart.

Yates plays for a former career backup quarterback, Gary Kubiak. Kubiak calls Yates “let’s be honest, a blessing” because he worked out over the summer with teammates at Rice University when coaches could not speak to him during the lockout. In Yates, then, Kubiak may have uncovered a quarterback in his own image.

“I knew I had to prepare hard mentally because I wasn’t going to get the reps physically,” Kubiak said. “I also understood that nobody would know if I was doing my job until I got thrown into the fire. My job was to prepare away from practice. You just know you’re taking on a big responsibility when the team loses a leader and starter. You’ve got to be big enough to handle that. That’s probably the biggest trait you’re looking for in a backup quarterback.”

Carr watches Yates from a distance and is delighted by his success under what he thinks is the most problematic backup situation: a young player who was never meant to have to play being suddenly thrown into meaningful action. He knows there must have been some nervousness in the Texans’ locker room when an untested rookie had to play. He knows because he feels it, too.

“It’s funny when the backup quarterback goes in — everyone panics,” Carr said. “I’m the same way. I see a backup go in and I think, Oh great, now what are they going to do? I’m speaking for a lot of backup quarterbacks in the league, we have a lot of confidence in ourselves. If I could reassure everyone, we know what we’re doing.”