The Seattle Mariners have not been very good for a while. They last posted a winning record in 2009 and haven’t cracked 90 wins since 2003. That being said, since 2005 the Mariners and their fans have had the privilege of watching Felix Hernandez pitch every fifth day. In 2013, however, the Mariners were fortunate enough to follow-up on each King Felix start with one from Japanese right-hander Hisashi Iwakuma. And how good was Iwakuma this year?

While the traditional Wins Above Replacement (WAR) stat has been based off of Fielding Independent Pitching (FIP – a statistic that is scaled like ERA and composed of the three outcomes a pitcher can truly control: home runs, walks, and strikeouts), we can also examine pitching performances with a stat called RA9-WAR, which uses runs allowed per 9 innings (RA/9) as its basis rather than a modified version of FIP.

Traditional WAR is handy because it is based off of FIP, and it allows us to make predictions based on what we would expect to happen. Put more clearly, regardless of how many runs a pitcher allowed in his last three starts, if he’s striking out a lot of batters, not walking any, and keeping the ball in the yard there’s a pretty good chance he’s due for some positive results. He can’t control groundballs that sneak through the infield, inherited runners his relievers allow to score, outfielders who get poor jumps on fly balls, and so on.

Many reject the notion that we can just dismiss what happens when a batter puts the ball in play. They fail to accept the idea that a pitcher can’t forcibly induce weak contact or pitch specifically to his defense. However, these same people should be more receptive to RA9-WAR. Rather than trying to predict which outcomes a pitcher can control, we can evaluate pitchers based on what actually happened, which is basically what more widely cited statistics like ERA and WHIP do.

So who led each league in RA9-WAR in 2012? Turns out Clayton Kershaw and RA Dickey went 1-2 in the NL, and Justin Verlander and David Price in the AL. Who won the NL Cy Young last year? Dickey, in a close vote over Kershaw. And in the AL? Price, in an even closer vote over Verlander.

This is not to necessarily say that anyone who voted for the Cy Young Award in either league is even remotely aware of what RA9-WAR is or means. But RA9-WAR passes the eye test – pitchers who tend to rate highly by this metric also happen to be really good.

Anybody want to take a guess as to who led the AL in RA9-WAR in 2013? You guessed it, the 32-year-old Japanese import with a fastball that averaged under 90 mph.

Shocking? Perhaps it shouldn’t be. His 3.16 ERA and 4.15 FIP from 2012 seem underwhelming, as does his 19.5% strikeout rate and 8.5% walk rate. But considering Iwakuma dramatically improved (2.65 ERA, 1.23 WHIP, 20% K rate and 7% walk rate in 95 innings) after being moved from mop-up duty out of the bullpen to the starting rotation, maybe we could have expected he would be one of the league’s more valuable pitchers if given the opportunity to throw 180-200 innings.

How has Iwakuma done it? Well for starters, he’s absolutely dominated hitters after getting ahead in the count. After throwing a first-pitch strike, Iwakuma held batters to a .201/.226/.312 slash line. He struck out 28.7% of batters after getting ahead and walked just 12 batters, recording a K/BB ratio of 10.58.

The key to Iwakuma’s success? His off-speed arsenal. Armed with a slider he threw to predominantly right-handed batters and an outstanding splitter, Iwakuma induced more swings at pitches outside the zone than any AL pitcher not named John Lackey. Below we can see the percentage of swings (left) and swings & misses (right) on pitches in and out of the zone (courtesy of Brooks Baseball):

As you can see on the left, Iwakuma generated nearly as many swings down and away from left-handed batters (these plots are from the catcher’s perspective) as he did on pitches right down the middle. On the right, we see that Iwakuma induced a ton of whiffs on pitches in the dirt.

In GIF form (courtesy of Fangraphs):

But what about Iwakuma stood out most of all? Simply put, his ability to prevent runs from being scored. He was especially outstanding when it came to pitching with men on base. Take a look at this AL leaderboard showing wOBA allowed with men on base:

Hisashi Iwakuma: .248 Felix Hernandez: .266 Hiroki Kuroda: .275 Max Scherzer: .277 Ivan Nova: .277

Iwakuma excelled at preventing damage in the most damaging situations. Of the 25 home runs he allowed, only 6 came with men on base.

Many reject the idea that we can use statistics like FIP and WAR to predict future outcomes. But when it comes to evaluating past performance, we have no choice but to give Iwakuma credit for what actually happened.

Iwakuma will probably get some well-deserved 4th and 5th place Cy Young Award votes, and stats like RA9-WAR show us he clearly was one of 2013’s most outstanding pitchers. Signed for 2014 at just $6.5M, the Mariners can look forward to 2014 knowing they have one of the game’s best values.