The transformation was visible to the naked eye.
In 2005, a career-changing season in many ways, Haren moved from St. Louis to Oakland, from sometime starter and part-time reliever for the Cardinals to rotation anchor for the Athletics. He was 24.
A star was born, but not until a significant alteration in his delivery literally gave Haren pause, enabling him to gather and compose himself before unloading his premium stuff.
"I began developing it in '03, in Triple-A," Haren said. "I did it a little in '04 when I was sent down [by the Cards]. Everything I do is slow. I try to keep it simple."
The hesitation comes midway through his delivery. It's unorthodox -- like a free-throw shooter pausing before his release -- but it is one of the keys to Haren's success.
"It's to keep myself back," he said. "I had a problem as a young guy keeping ahead of myself. I started doing [the pause] in the bullpen, to keep myself behind the baseball. In games, I'd revert to my old self."
Learning to control the adrenaline, he took the hesitation delivery to the mound in 2005 after going west in a deal for Mark Mulder. With the youthful A's, Haren felt comfortable trying something new and sticking with it.
"Oakland let me go out every fifth day," he said.
From 46 innings with the Cardinals in '04, Haren jumped to 217 innings and 34 starts in '05, completing three games while going 14-12 with a 3.73 ERA.
He was on his way to becoming one of the game's most durable and successful pitchers, averaging 224 innings and 14.6 wins the next five years.
Haren started the All-Star Game for the American League in San Francisco in 2007 -- "probably the most nervous I've been." It turned out to be his final season in Oakland before getting dealt to Arizona for a collection of young players.
He flourished in his new league for 2 1/2 seasons before the Diamondbacks returned Haren to the American League West -- and home to Southern California -- last July in exchange for Joe Saunders and three pitching prospects.
Drilled on the right forearm by a line drive off the bat of Boston's Kevin Youkilis in his first start for his hometown team, Haren showed his toughness by rebounding quickly from what many feared was a serious injury.
"I don't think he told us anything with that," Angels manager Mike Scioscia said, "but you saw it firsthand. That was scary."
Dodging a bullet with a deep bruise on what could have been a season-ending fracture, Haren learned he was a quick healer.
"My first start here was the only time in my career I've been pulled from a game for an injury," Haren said.
Back for his next assignment, Haren pitched superbly down the stretch, finishing with a 2.87 ERA in 14 starts for the Angels. His 35 outings for the Diamondbacks and Angels combined represented a career high, along with his 235 innings.
"This guy has been a premier pitcher for a long time," Scioscia said. "He has a lot of things he brings to a pitching staff -- not only on the field but in peripheral areas, with his preparation and approach.
"He threw 235 innings last year; that saves wear on the bullpen. This guy wins. He's a terrific pitcher."
Jered Weaver, Haren, Ervin Santana, Joel Pineiro and Scott Kazmir give the Angels a rotation potentially as good as any in the American League.
Haren can anticipate a smoother ride than last year. His 3.91 combined ERA was higher than his 3.66 career norm, and his 12-12 overall record represented his lowest win total since he became a full-time starter.
When you consider that he was pitching for teams that combined to finish 34 games below .500, that 12-12 looks more impressive.
"The way I look at it, last year was kind of a worst-case scenario for us," Haren said. "Obviously, I wasn't here when Kendry [Morales] was hurt, but he was a big part of it. This organization hasn't been in this position for a while, so there's a lot of motivation.
"Having the rotation together for a full season should be a good thing. Guys are feeding off each other. I've been part of some good rotations, and this one ranks right up there. I think we'll give the team a chance to win.
"It all starts with pitching and defense. Look what it did for San Francisco last year. They didn't exactly score eight runs a game."
Haren's superb control with his full repertoire enables him to get early-count outs and advance deep in games. When he departs, he'll feel confident in the arms coming in behind him.
"We have plenty of guys who can get it done," Haren said.
There was no hesitation in his delivery.