As Patrick Corbin continues his tour of the I-95 corridor — if it was Philly on Tuesday and D.C. on Wednesday, it has to be New York on Thursday! — his suitors must decide how deep into nine figures they should plunge to sign the popular free-agent left-hander.

But Corbin's most recent pitching coach is here to predict it will be a worthwhile expenditure.

"I think that he should be able to maintain this level of excellence for a really long time," Arizona Diamondbacks pitching coach Mike Butcher said by phone Thursday. "He really should."

Corbin reached free agency after a career-best season in which he posted a 3.15 ERA in 200 innings, finishing fifth in the Cy Young Award voting. He struck out 11.1 batters and walked only 2.2 per nine innings and was second in the league with a 2.47 FIP, a metric that measures only a pitcher's ability to prevent home runs, walks and hit by pitches and cause strikeouts.

Indeed, Corbin should have replaced his warm-up song with the "cha-ching" of a cash register.

But while the nine-figure offers for Corbin will reflect a pitcher who appears to be peaking at age 29, his spotty history before 2018 should offer some pause. He is four years removed from Tommy John elbow surgery that caused him to miss all of the 2014 season. And from 2012-17, he had only one season with a sub-3.50 ERA and fewer than 1.20 walks/hits per inning pitched. Before this year, his career high for strikeouts per nine innings was 8.4.

Yet Corbin could be in line to top the six-year, $126 million deal that Yu Darvish signed with the Chicago Cubs last winter, too rich for a Diamondbacks team that would love to keep him. And with the Yankees — Corbin's favorite team while growing up near Syracuse — in the mix, the Phillies might have to overpay.

Butcher has worked with Corbin since 2016, when the lefty posted a 5.15 ERA and got booted to the bullpen for the final six weeks of the season. He contends that Corbin has been building toward this level of success.

"Going back to '16, it was more of him feeling himself out and getting back to being healthy," Butcher said. "In '17 and '18, it was really the evolution of a change of speeds on his slider."

Patrick Corbin, left, with Arizona Diamondbacks pitching coach Mike Butcher.
ROSS D. FRANKLIN / AP
Patrick Corbin, left, with Arizona Diamondbacks pitching coach Mike Butcher.

Corbin's slider has long been one of the nastiest pitches in baseball, but he lacked more than a fastball to offset it. By the middle of the 2017 season, it became clear to Butcher that Corbin needed to give hitters a different look.

After trying unsuccessfully to change speeds on the slider and experimenting with a change-up, Corbin finally developed a slow curveball that he began throwing midway through the 2017 season. Butcher calls it "a game-changer."

"When you're throwing 93 [mph] and a hard slider, the curveball acts as a change-up as well as a different shape of a pitch," Butcher said. "When it's coming out of the same slot as the slider, it's very deceptive. The curveball became the pitch where he could get ahead in counts, he could steal a strike. It was a big pitch for him."

In fact, it turned Corbin from a middle-of-the-rotation starter into an elite one. But any team that decides to make him the 22nd pitcher ever to receive a nine-figure contract is betting on 2018 being the start of his peak, not the height of it.

Phillies president Andy MacPhail and general manager Matt Klentak prefer to develop young pitching and spend big money to acquire hitters. That's typically a more prudent path.

But Butcher noted that Corbin put less strain on his arm because he couldn't throw as much as his peers who grew up in more conducive climates than frosty upstate New York. That didn't prevent Corbin from tearing an elbow ligament early in his career. He has, however, thrown only 945 2/3 innings in the majors. Darvish, by contrast, had logged more than 2,000 innings between Japan and the majors before signing with the Cubs at age 31.

"There's not a whole lot of wear and tear on his arm because he didn't pitch a whole lot in his youth, so he didn't get beat up that way," Butcher said. "He takes care of his body. He eats right. He does all the necessary things to keep himself on the field. There's nothing I can say bad about the kid. He's a pro. Everything you want out of a pitcher, he is."

At what cost, though?

The Phillies are about to find out.