Shuffling quarterbacks leaves some teams without a full deck

There are 6.7 billion people in the world.

The NFL has only 32 teams.

So where are all the good starting quarterbacks?

NASA had an easier time narrowing down the seven original Mercury astronauts than pro football has had finding Sunday signal callers with the right stuff.

For evidence of just how difficult it is to predict the success of NFL quarterbacks, look no further than today’s three games: Tennessee at Detroit, Seattle at Dallas and Arizona at Philadelphia.

Each of those six teams has adjusted their quarterback situation this season because of either injuries or poor play. Some have gotten it right; others are still groping in the dark.

The Titans and Cardinals have benefited greatly from the decisions to start Kerry Collins and Kurt Warner (combined age 72) and took risks in the process by benching Vince Young and Matt Leinart, both top-10 picks in 2006.

The Titans sat down Young after Week 1, whereas Leinart was demoted to backup at the end of Cardinals training camp.

Dallas didn’t know how much it would miss Tony Romo until he was gone, sidelined by a broken pinkie. When he was out, and Brad Johnson took his place, the Cowboys went from averaging 29.1 points to 13.7.

Terrell Owens said he felt “unleashed” in Sunday’s 35-22 throttling of San Francisco. Think that had anything to do with Romo? Playing his first game at Texas Stadium since Oct. 5, he put up a stat line fatter than Jerry Jones’ billfold: 23 of 39 passing for 341 yards and three touchdowns -- to three different receivers.

“We put ourselves in position to make a run,” Romo said. “The time is now. We’ve kind of gotten ourselves in a spot where we’re in a hunt for a playoff spot. It’ll be exciting. You never know what’s going to happen.”

The Seahawks can attest to that. Who could have guessed they would drop off so sharply after years of owning the NFC West? A big reason for that decline is the struggles they’ve had at quarterback, where Matt Hasselbeck has been hobbled all season. Back problems have kept him out of five games, and those issues have led to knee problems.

Seneca Wallace started in place of him and, for the most part, did a respectable job. Still, losing a three-time Pro Bowl quarterback can quickly trigger a tailspin.

You want unpredictable? Look at the Eagles. Andy Reid’s confidence in Donovan McNabb has gone up and down, up and down, and now -- fittingly -- it’s the coach who looks like a yo-yo.

The struggling McNabb was benched Sunday for the first time in his 10-year career, mummified on the sideline in the second half at Baltimore. As the TV cameras zoomed in, the only Eagles player who looked more lost was his replacement, Kevin Kolb, who had the pleasure of coming in cold against an attack-dog Baltimore defense.

It would get uglier. Kolb was intercepted twice -- once for an NFL-record 108-yard return for a touchdown -- then picked off again the next day when Reid sent him back to the bench. McNabb will start tonight, and the Philadelphia crowd might make him long for Baltimore. Look for McNabb and Reid to be packing their bags after the season.

Maybe McNabb can reinvent himself with another team, perhaps a place such as Chicago or Kansas City or Minnesota. Hey, Warner did that after flaming out with the New York Giants. Now he’s producing the type of numbers that have put him squarely in the most-valuable-player discussion.

Warner is 37. That happens to be the same passer rating -- 37.7 -- that Detroit’s Daunte Culpepper wound up with Sunday, when the Lions blew a 17-0 lead on their way to a 38-20 loss to Tampa Bay.

Hard to fault Culpepper. He was retired, out of the game for nearly a year when the Lions signed him in desperation early this month. They had lost Jon Kitna for the season, then turned to the untested Dan Orlovsky. When Culpepper hit a rough patch against the Buccaneers, Detroit tried out Drew Stanton. Culpepper returned after Stanton suffered a concussion.

The Minnesota Vikings’ love-boat scandal behind him, Culpepper is now a deckhand on the S.S. Minnow.

After his team dropped to 0-11, Culpepper had some upbeat words that, read the wrong way, could sound ominous. “There’s no way anybody around here should be like, ‘Hey, it’s over,’ ” he said. “No, it’s never over.”

Now that’s scary.

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sam.farmer@latimes.com