The knock on Clayton Kershaw was always his control. He couldn’t throw some of his pitches for strikes and was prone to wild spells. He has pitched so spectacularly this season that some may not remember that critique.

In 198 2/3 innings, Kershaw is still striking over a batter per inning, but his walk rate has dropped to slightly over six percent of opposing hitters. While that rate isn’t in the top ten of the National League right now, it represents a marked improvement. From 2008-10, Kershaw walked 11.1 percent of the opposition, ranging from 11-13 percent over the three year span. Among senior circuit pitchers with 400 innings pitched in the span, only Jonathan Sanchez issued free passes with a greater frequency.

Missing bats was never a problem, as his career 9.4 K/9 will attest, but his filthiness proved detrimental to his command. He had the stuff to succeed but had not yet harnessed it. Thus the walks. Now those walks are a thing of the past and is he surpassing what many believed to be his potential at the ripe age of 23 years old.

He entered the league with a world-renowned curveball but couldn’t throw it for strikes. His changeup had potential but it wasn’t a true out pitch. Kershaw threw his fastball with the best of them and could induce whiffs on his offspeed pitches, but he struggled to throw them for strikes. He wasn’t going to blossom into an elite pitcher with a fastball-average curveball-below average changeup repertoire. As hitters began to notice he struggled to control the pitches, they began to lay off, and something had to give.

Kershaw took that opportunity to fine tune his slider. He began throwing the pitch last season, reducing his usage of curveball from 17 percent to just 7 percent. He also got rid of the changeup, throwing it just two percent of the time. For the most part, Kershaw was a fastball-slider pitcher last season, and the slider proved devastating. Whether you trust our pitch run values or not, it’s clear that his slider produced some of the best results of any single pitch last season. Whether using overall runs saved or runs per 100 pitches thrown, he held a clear lead over all other slider-throwers.

The pitch grew even more effective this season, perhaps as he better understood how to sequence and locate it. Kershaw also began to work the changeup in more, giving him an upper echelon fastball-slider-changeup repertoire. His BB/9 dropped from 4.8 to 3.6 last season and he sustained the high strikeout rate. Add to that more grounders than flyballs and Kershaw was well on his way towards becoming one of the best pitchers in the game. Not one of the best young pitchers, or a nice pitcher who could get there in a few years, but one of the best overall pitchers in baseball, free of detracting qualifiers.

Quite the transformation.

At 23 years old, Kershaw is an elite pitcher. He has also garnered plenty of recent support for the Cy Young Award, which seemed like Roy Halladay‘s to lose for most of the season. Kershaw’s age and quick jumps from solid prospect to good major league pitcher and then to elite starter place him in some limited company. Many fans and analysts are quick to point out how rare Justin Upton’s accomplishments are given his age, but Kershaw is deserving of a similar treatment. How many pitchers, 23 years old and younger, have experienced the same type of success?

Going all the way back to 1901, and requiring at least 600 innings logged before turning 24 years old, Kershaw has the 10th best ERA+ at 131. If we instead go back to 1950, then only Dwight Gooden, Bert Blyleven and Dean Chance rank ahead of him. Kershaw finds himself in even more limited company when some of his best attributes are introduced. Since 1901, there are only two pitchers to throw 600 innings before their 24th birthday, while also posting a 9+ K/9: Kershaw and Sam McDowell. The latter whiffed 9.7 batters per nine innings over 799 2/3 frames over the 1961-66 seasons.

Very few pitchers experience Kershaw’s success. Not only are there fewer pitchers called up at such a young age, but it generally takes time for certain youngins to mature at the major league level. Kershaw is a big-bodied lefty flamethrower with likely the best slider in baseball, who now strikes four batters out for every walk. He might not yet be the best pitcher in the National League, but he is making a case as the man to dethrone Roy Halladay over the next few seasons.