On Monday night in Baltimore, Rick Porcello allowed two earned runs in a complete-game 5-2 victory, striking out seven without a walk. That outing lowered his season ERA to 3.08, second in the American League only to Masahiro Tanaka. It raised Porcello's innings total to 210 2/3, putting him among only five AL pitchers with 200 innings, and he'll get the chance to increase that Saturday evening against Tampa Bay. It improved his record to 21-4 -- which, I don't need to tell you how poor of an evaluating tool a win-loss record is, and a big part of that is Porcello getting an absurd 6.7 runs per start of support from Boston's offense, but there's a part of me that refuses not to be at least a little impressed by 21-4.
Porcello, over the past month or so, has gone from fringe AL Cy Young Award candidate to a legitimate possibility, perhaps even the front-runner. The argument is right there if you want to make it. Sure, you could probably make the argument that Porcello's ERA is more a product of good fortune than performance by pointing to his .260 Batting Average On Balls In Play, which is 42 points lower than his career mark. But then also you've got to consider that his career mark is probably unfairly inflated by his being a ground-ball pitcher in front of Detroit's usually below-average infield defense for so many years, and that the BABIPs of his strongest AL Cy Young Award competitors are similarly depressed.
• FanGraphs' Craig Edwards looks at the impressive debut of Alex Reyes
So you could make the case that Porcello's numbers point more to good fortune than performance, or you could make the case that Porcello has made some legitimately compelling strides in the way he pitches.
This winter, FanGraphs' Jeff Sullivan noted that, late last year, Porcello had morphed his curveball into a near-replica of the famous hook thrown by Adam Wainwright. Back in May, I wrote about how Porcello seemed to finally be learning the best mix of his two-seam and four-seam fastballs, and how to properly use the latter.
My tagline for that piece was, "When heightening the quantity of the four-seamer backfired, he heightened the quality instead." In short, Porcello threw his four-seam fastball more than ever in 2015, his first season in Boston, at times throwing it more often than his trusty sinker. Porcello gets some of the best four-seam spin in the game, so it was an intriguing idea, but the results were bad. In May of this year, Porcello had more or less reverted to his typical four-seam usage rates, except with a twist: an unprecedented percentage of his four-seamers were coming in two-strike counts, and he was elevating them to get the most out of the spin, and it was working. The two-seam got him ahead, and he used the four-seam only when he needed to, to put batters away. Quality over quantity.
Except now, as Porcello's morphed from fringe to legitimate AL Cy Young Award contender, there's been another development in the fastball department. It looks like this:

The quantity is back. That's Porcello's four-seam usage rate, by month, since the beginning of the season. Since I wrote that article in May, Porcello has raised his overall four-seam usage rate from 14.9 percent over the first month and a half to 27 percent in September. In his Sept. 14 start against Baltimore, he led with it (38 percent), throwing 18 more four-seams than sinkers.
And Porcello has increased that usage in a very particular way. Early in the season, he figured out that his four-seam was best used as a two-strike weapon. Lately, Porcello has figured out another way to use it.

Porcello's usage rate against righties hasn't changed nearly as much, fluctuating between 12-20 percent for much of the year. No, instead, the added four-seamers have come almost exclusively against left-handed batters, as he has nearly doubled his usage rate against lefties with the pitch since the beginning of the season.
And while the two-strike fastball was an early-season development, it's not been until recently that Porcello has started going to the four-seamer early in counts, too. Going back to that Sept. 14 start against Baltimore, in which Porcello threw eight innings of one-run ball, here's how he started off the 12 lefties he faced:
Changeup
Four-seam
Sinker
Four-seam
Curveball
Four-seam
Four-seam
Four-seam
Four-seam
Changeup
Changeup
Four-seam
This is not the Porcello we've grown to know. This is a new Porcello, with whom we ought to become acclimated.
And it's not just the way Porcello uses his four-seamers against lefties that's changed. It's the way he's using the two-seamer against lefties, as well. The lefty four-seamer has changed in frequency. The lefty two-seamer has changed in location:

In his early Tigers days, Porcello hovered around a quarter of his two-seam fastballs coming inside to lefties. As time went on, that gradually increased, but it's now spiked during his time in Boston, and nearly half of Porcello's two-seam fastballs to lefties are in on the hands. Why throw the two-seam inside to lefties? Here's Porcello's career BABIP against lefties with the two-seam, broken into zones:

Throughout his career, when Porcello has gotten the two-seamer inside on lefties, the BABIP's been just .255. Porcello is throwing the two-seamer inside now more than ever. That career-low overall season BABIP is starting to feel a little less like luck and a little more like process.
Add it all up, and Porcello's results against his opposite-handed foes are particularly compelling:

It's career bests across the board, in some instances by laughable margins. The last two columns being the most important, and also perhaps the most striking. A touch over 100 innings is perhaps too small a sample to declare that Porcello has figured out lefties, but it sure looks like Porcello has figured out the lefties who have plagued him throughout his career. The formula? More four-seam fastballs than ever, particularly early in the count, and more two-seamers inside for weak contact.
Changing the way Porcello uses his fastballs against lefties has helped shore up the biggest weakness of his game. Shoring up that weakness has helped him average more innings per start than all but four pitchers this season. Getting the most out of both quality and quantity has turned Porcello into a legitimate AL Cy Young Award contender.
A version of this article first appeared at FanGraphs.com. This story was not subject to the approval of Major League Baseball or its clubs.