Just as Stephen Strasburg prepares to sit down, Ross Detwiler looks like he’s ready to stand up to his full height. And that may prove to be quite tall.

The 6-foot-5 southpaw, gifted with what old-timers praised as “the easy gas,” has emerged since the all-star break as one of the three hardest throwing left-handers in the National League, a tick behind the Nats’ Gio Gonzalez and 2011 Cy Young Award winner Clayton Kershaw.

With 10 straight powerful starts since the midseason break, Detwiler has gained more and more confidence in his stuff, his heat trending higher as the season progresses, until now he has also moved into baseball’s elite in nearly every statistical category.

After seven scoreless innings in a 2-1 win over the Cubs on Labor Day (in which he barely labored at all), Detwiler is now in MLB’s top 20 in ERA (3.15), hardest to hit (batting average), hardest to reach base (WHIP), fewest pitches per inning and most groundballs.

If this is who Detwiler has truly become, and his ERA is 3.10 over his last 206 innings, encompassing all of ’11, too, then the Nats have found a lefty comparable to, or a hair behind, Gonzalez and Jordan Zimmermann.

Couldn’t come at a better time. Detwiler is the Nats’ presumptive fourth starter to join their postseason rotation with Strasburg getting moth-balled.

The 26-year-old, who has spent the last five years battling changed mechanics, hip surgery, a lost curveball and “zero confidence,” has picked a perfect time to put the pieces of his sixth-overall-draft-pick talent back together and emerge as one of the game’s rising stars.

“Det was awesome. He’s been pretty awesome all year long, but today he was real special,” Manager Davey Johnson said.

Above Detwiler’s locker is a poster from the action comedy cop movie “The Other Guys” with Will Ferrell and Mark Wahlberg flying through the air, dressed in suits, but karate kicking while firing a pair of pistols apiece. But Detwiler and Edwin Jackson’s faces have been photo-shopped in place of the movie stars, courtesy of Jackson’s fiancée.

“Which one am I?” Detwiler said. You’re Ferrell. Finally. Be happy.

On Labor Day, the Nats faced, and beat, Cubs right-hander Jeff Samardzija, who ranks third in baseball in velocity behind only Strasburg and Tampa Bay ace David Price. The big Cub topped out at 98, Detwiler at 97.

Detwiler’s delivery is graceful, flowing, almost languid, until his rising fastball zooms through the top of the strike zone at 94 to 97 mph, or his sinker, an even tougher pitch, dives at hitters’ knees just a blink less quickly. No other pitcher in baseball can throw his two fastballs so frequently, 80 percent of the time, simply daring hitters to cope with them and yet still flourish. In fact, when the Nats coaches encouraged him to trust his smoke after the break, he went fastball crazy, throwing 89 percent heat in nine straight starts, including 155 fastballs in 162 pitches in back-to-back starts.

The Nats’ brass had to chuckle. Was this the same choir-boy Detwiler who looked like he was 12 when he was drafted, whose speed dropped 4 mph and whose confidence collapsed after the Nats stupidly changed his cross-the-body throwing mechanics and almost trashed his career. Was this the lefty who lost his fabulous college curveball and, to this day, has never rediscovered how he threw it?

“I wish I could put out an APB on a scouting video from USA Baseball in ’06,” Detwiler said. “That may be the only piece of tape left anywhere that shows me throwing that curveball.”

How good was it? “I threw one that started in the lefthand batter’s box and ended up down-and-in to a right-handed hitter,” he said. “Pretty good.”

Finally, the Nats’ brass, including GM Mike Rizzo, threw a collective rant and said, “Where the hell is the guy we drafted? Get him back.” The Nats, who’d once told Detwiler “you’ll never pitch in the majors throwing that way,” told him “go back to throwing that way.”

He’d forgotten how. “I have no idea what my old mechanics were,” he says. “I rebuilt it and now I’m finally comfortable again.”

Now, even Detwiler’s own teammates can hardly believe how far he’s come even though, in the last two Nats spring trainings, the buzz was always, “Who’s throwing the best of anybody? Detwiler. Go look at him.”

“We were in the bullpen looking at the [radar readings],” Tyler Clippard said. “We were trying to figure out what left-handers throw harder. Price and Matt Moore in Tampa Bay. Who else? Basically, nobody.”

That’s correct, give or take six inches, especially if you look at his last 10 starts, including Monday’s 94.0 average speed.

“Det is worlds apart from where he was at the start of last year. Now, he looks like he has all the confidence,” Clippard said. “He’s spotting his sinker at 95. I’d put him up with anybody.”

The last (huge) piece of the puzzle came this year: mound presence. From Gonzalez he took the lesson to be happier in his work, less tense. But, during a bullpen stint, it was veteran lefty Mike Gonzalez who preached “that aggressive attack attitude,” Detwiler said. Now, before his first pitch, his intro music is Metallica’s thundering “Wherever I May Roam.” Come on, adorable skinny Ross Detwiler?

“After I hear that song, I get a huge adrenalin rush,” Detwiler said. “My first three pitches of the game were sinker for a called strike, fastball up and away for a swinging strike and a sinker painted low away [at 95] for [called] strike three. I think that sent a message.”

The Nats’ scoreboard advised the crowd: “Someone or something effecting your enjoyment of the game? Call . . .” Too bad the Cubs couldn’t complain about Detwiler spoiling their fun.

The day’s best development was that Detwiler incorporated the advice of pitching coach Steve McCatty to use his change-up and curveball more as the game progressed and hitter’s got a better bead on his fastball. Detwiler ended minor jams in the fourth, fifth and seventh by getting the last outs with a curve, change-up and curveball for a double play, all pitch calls by catcher Kurt Suzuki. Still, 78 of his 93 pitchers were two varieties of that easy gas.

Years ago, when the Nats introduced Detwiler, General Manager Jim Bowden gushed about how power-pitching left-handers were essential for “postseason baseball.” The Nats did everything but name Detwiler their Game 1 starter in the World Series — year or decade unknown. Bob Boone compared him to Steve Carlton as a skinny teenager.

“I remember — July 5, 2007,” Detwiler said. “I’ve been through a lot since then, including one point with zero confidence.”

Now, it’s come full circle.

“Left-handers take longer. Sometimes, they have to figure out how good their stuff is,” Johnson said. “Koufax didn’t put it together until he was 26.”

Oh, give it a rest.

Johnson gives a you-never-know-how-good-they’ll-get shrug. “When he learns how to use his whole arsenal, it’s going to be a lot.”

A lot of what?

Fun, probably.

For previous columns by Thomas Boswell, visit washingtonpost.com/boswell.