NASHVILLE, Tenn. (AP) The adrenaline spikes the instant Charlie Whitehurst sees the starting quarterback hurt and walking to the sideline.
''All of a sudden, it's like, `Here we go. Where's my helmet?''' Whitehurst said. ''That's the first thought that goes through my mind. `Who's got my helmet?'''
It's always Next Man Up in the NFL, yet the backup quarterback has both the easiest job in the league and the toughest. Collect a paycheck for standing on the sideline and wearing a cap game after game. Then at the drop of a starting quarterback, be ready to immediately step in and direct an offense and throw touchdown passes while the best defenders on the planet are trying to tackle you.
Coaches constantly are searching for the next starting quarterback, or a cheap rookie with potential, which makes it even harder to survive as a long-term backup. Whitehurst is called ''Clipboard Jesus'' because the long-haired, bearded Clemson player has spent most of his pro career holding a play sheet.
He is now in his ninth season with his third team, a surprisingly long career for a player never considered a true starter.
Baltimore offensive coordinator Gary Kubiak played nine seasons in Denver backing up John Elway. Dallas head coach Jason Garrett played 12 seasons, most spent backing up Troy Aikman with the Cowboys.
Luke McCown is in his 11th season, now behind Drew Brees in New Orleans. Who holds the mark as the longest-tenured backup QB isn't a stat that's officially tracked, though Pat Ryan lasted 13 seasons, most with the Jets, never starting a game.
Matt Hasselbeck started in the NFL backing up Brett Favre in Green Bay and now is behind Andrew Luck in Indianapolis. The veteran says being a good backup quarterback really depends on the team and the starter.
''When I came in with Green Bay, Brett Favre was coming off his third consecutive MVP, so for me, I was trying to learn his intangibles, his leadership style, copying his verbiage,'' Hasselbeck said. ''It was everything, the way he did it and then being ready if he got hurt. And then the coach has to be comfortable that you can go in and play.
''In other situations, like Tennessee, with Jake Locker, he just needed a little experience, so your position is a little different in that regard. You've still got to be ready to play, but you're talking more about football and philosophy and you have more dialogue. In Green Bay, there was basically no dialogue.''
Locker is in the final year of his rookie contract, so the Titans could afford giving Whitehurst a two-year deal worth up to $8 million with incentives. Coach Ken Whisenhunt worked with Whitehurst last year in San Diego and wasn't worried that the quarterback hadn't thrown a pass in a game the past two seasons. Whisenhunt wanted someone who could help teach his offense to a young quarterback.
''He understands what we're trying to get done, so he can operate with fewer reps just because he's seen a lot of this before,'' Whisenhunt said. ''That's definitely a valuable asset.''
To Whitehurst, it's simple.
''I try to do everything they ask me to do as well as I can,'' Whitehurst said.
Starting quarterbacks get most of the work in practice, while the backup usually runs the scout team. The backup has to work harder to learn his team's offensive plan each week, preparing each day as if he will start, even though coaches and first-teamers ignore him.
Whitehurst said he has worked on staying ready, but relaxed, since he was in college.
''I'll drive myself crazy if I'm game mode all the time,'' Whitehurst said.
Playing quarterback in the NFL is almost a family business. Whitehurst's father, David, was a quarterback with Green Bay between 1977 and 1983, and in 1984 with Kansas City.
Jacksonville coach Gus Bradley said Whitehurst has the ability to rally a team. Bradley was with Whitehurst when the Seahawks needed to beat St. Louis at the end of the 2010 regular season to win the NFC West. Whitehurst started and won 16-6.
''It's obviously not too big for him, and you're not going to face a quarterback that's going to be in the situation for the first time,'' Bradley said. ''He's been in the league a long time, so I think he will bring calmness to their team.''
With Locker battling injuries, Whitehurst has had three starts in the past four games. He already has career highs in yards passing (764) and touchdowns (five).
But it's usually not good if the backup starts too much. Nine of the 12 playoff teams in 2013 had one quarterback start all 16 games, according to STATS.
''There's only so many jobs, and the guys that have them have earned them,'' Whitehurst said.
AP Sports Writer Michael Marot in Indianapolis and Dave Ginsburg in Baltimore contributed to this report.
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