Offensive improvement in 2012 will depend largely on how well QB Russell Wilson does on third down. / Drew Sellers, Sportspress Northwest

No single reason explains why Russell Wilson is the starting quarterback of the Seahawks. But there is a single stat from a single game that explains a lot about why Tarvaris Jackson is no longer the Seattle QB:

In the final game of the 2011 season against Arizona, the team the Seahawks play in their opener Sunday, the Seahawks were 3-for-19 in third-down conversions. Arizona won, 23-20, in overtime.

Buh-bye, T-Jack.

The single biggest metric that will determine whether Wilson will stay as starting QB is improvement on third downs. Obviously, the task isn’t solely his responsibility, but he’s the cowboy in the saddle in charge of getting the herd to market and staying off the barbed wire.

And he’s a rookie. The Cardinals defenders are not.

Third-down conversion isn’t necessarily the best predictor of wins. But the Seahawks finished 25th in the NFL (38 percent) in that category primarily because Jackson wasn’t a good decision-maker when the field and clock shrank. The shortcoming may not have seemed so bad because they played four games against the NFL’s two worst teams in that department — No. 31 Arizona (30 percent) and No. 32 St. Louis (15 percent — yeesh).

But the Seahawks had one of the NFL’s best running backs, Marshawn Lynch, who on that Jan. 1 game against the Cardinals had 86 yards in 19 carries, and they still couldn’t move the ball in the crucible.

Part of the low overall ranking was due to the post-lockout hash of an offense under newbie Jackson that was the biggest reason for a 2-6 seasonal start.

“It was most unfortunate to start (2011) with so many brand new young guys,” coach Pete Carroll said Wednesday. “Relatively, we’re in much better shape. There’s so much difference — communication, ability to adjust, the handling of different situations and issues. And depth-wise we have guys who can step in.”

The only problem is that the guy on the field in charge has never faced a pro defense that is loaded with talent and enlightened by a pro scouting report on him.

Impressive as Wilson has been, starting a rookie at QB is a risk that is on the edge of the NFL solar system, even if it is trendy. Amazingly, four other teams are doing the same — Andrew Luck in Indianapolis, Robert Griffin III in Washington, Brandon Weeden in Cleveland and Ryan Tannehill in Miami. All were first-rounders; Wilson was a third-rounder, the 75th player taken.

That’s the most rookies at the helm at least since 1950. The previous high was three.

And since 1970, Wilson is one of only six rookies drafted in the third round or later to start right away, none since Kyle Orton in 2005.

What this means is there’s ridiculously few precedents for what the Seahawks are about to attempt. Especially against a defense featuring Darnell Dockett, Adrian Wilson and a bunch of other gnarlies who eat railroad ties for appetizers.

“These guys are a very aggressive attacking group –  they would rather blitz you than anything,” Carroll said. “It’s going to be a constant. (Protecting Wilson) would really come on the shoulders of the guys up front to pick it up. We’re so much further along than we were at any time last year. I hope we do a much better job at targeting and identifying their schemes. It’s going to be hard for every quarterback.

“Hopefully he’ll be able to handle it.”

Impressive as Wilson has been in the preseason — 13 scores in 21 drives, and only one turnover –  and as intriguing as his future, the present means converting third downs Sunday against a force he’s never experienced.

“Doesn’t matter to us who’s playing (QB),” said Arizona coach Ken Whisenhunt, speaking as if he were a Steelers assistant coach, which he was. “We’re preparing based on what we think we need to do to try to be effective.”

Wilson’s career arc is becoming a wonderful novel. But every good story has setbacks for the protagonist, and Wilson is about to experience his first one as a pro.