Mitch Keller

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Today is going to be Mitch Keller’s first start of the season, and he is going to have quite a lot of eyes on him. The right-hander leads the list of top 10 Pirates prospects after his blazing performance in AAA in 2018 which anticipated a bright future for the young player.

2019? Not too pretty if you look at his regular numbers: in 48 IP, Keller had a miserable ERA of 7.13, winning 1 game and losing 5 in 11 games started, really forgettable stuff. But Keller could be the textbook case for embracing different metrics to evaluate pitchers.

To get our feet wet, let’s talk about some stats, namely wOBA, BABIP, xFIP, SIERA and (k-bb)/ip.

wOBA: I’m not going to get into the specific formula for it (you can check out a nice explanation here or even better, here) as it might make it look overwhelming when it is really not, but we can understand WOBA as a batting average on steroids (I believe we are already past the “too soon” period).

In the beginning, there was Batting Average (AVG), and it was good. But then we realized that AVG was flawed: it did not account for all the other ways a batter could get on base (walks mostly, but others too) so we got On Base Percentage (OBP) which measures the rate at which a batter gets on base not only by the way of hits but also incoporating walks and hits by pitch. And that seemed good.

We also got Slugging (SLG) and then we could acknowledge that not all hits are valued equally: it’s not the same to be a .350 AVG batter a-la Ichiro who mostly hit singles than a .247 AVG ball crusher like Khris Davis (he did it 4 seasons in a row!), that’s why there is an almost .100 difference in SLG between them. Then, OPS (OBP plus SLG) was born to try to have a unified stat which summarizes most of the offensive aspect of players, and now, everything looked definitely good.

But instead, Tom Tango, Mitchel Lichtman, and Andrew Dolphin introduced us to wOBA, and in their own words:

Do we really need another statistic? Yes, we do. Instead of trying to take two statistics (OBP, SLG) and combine and correct their flaws in the hopes of getting one number, we prefer to start from scratch…. wOBA is weighted on-base average (we call it an average rather than a percentage). When you look at wOBA numbers throughout the book, just think OBP, and you’ll be fine. In other words, an average hitter is around 0.340 or so, a great hitter is 0.400 or higher, and a poor hitter would be under 0.300.

Also, they said:

“…if you see a guy with a .360 OBP and a .360 wOBA, then you know he’s got a “normal” profile of extra-base hits and walks.  If you see a guy with a .340 OBP and .360 wOBA, then you know there’s a lot of power that his OBP is not capturing.  If you see a guy with a .380 OBP and .360 wOBA, then you know he’s not a power hitter.

And then, everybody went to rest. Well, at least until wRC+ was created, but that’s for another day.

Batters killed Mitch Keller producing a .392 wOBA against him when the league average was .320, so there is also an advance metrics where Keller looks bad if we stopped there. But we are not, so let’s look at these other numbers:

2019Pirates (AAA)75103.20.85270.2023.563.87

At first glance, there are a couple of things that got my attention: for once, the ERA estimators xFIP and SIERA are way lower than Keller’s ERA, 3.47 and 3.78 respectively, that’s quite a gap and tells us that something went weird for Keller to have been punished with that ERA when xFIP and SIERA tell another story. I tend to align with SIERA instead of xFIP for cases when we evaluate less than 200 innings pitched, so an ERA around 4.00 should be more realistic.

The other pretty telling thing that we can see in the table is that ridiculous BABIP of .475 that batters did against Keller, when the league’s average was .298. BABIP is the average that batters hit against him for balls in play excepting homers; it is influenced by three main factors: Defense, Luck, and Talent. As you can see 2 of those 3 things are completely out of the pitcher’s control, so we can assert that he was very unlucky (bloopers that fall right behind the fielder, a line drive hit where the fielder was initially but had to move from to keep a runner near a base, etc).

Also, guess which team was the worst rated by most of the defense metrics during the 2019 season? It starts with Pi and ends with Rates; it’s estimated that around 5 of their 93 division-leading losses could be attributed to their awful defense.

So we get to the Talent part. Keller is a very talented pitcher, last season his two more effective pitches were the slider and the curveball: he got swinging strikes from them 48% an 34% of the times, batters only producing a wOBA of .217 and .207 against those pitches, respectively.

What happened, then? Well, without any sugar-coating, his fastball was crushed. Batters slugged .719 against it (as opposed to .289 and .133 against the slider and the curveball) and also had an absurd .499 wOBA when facing Keller’s four seamer.

His fastball’s speed and spin rate are almost identical to Max Scherzer’s: around 95 mph and 2475 rpm, more than respectable. A big difference is that while Scherzer used it 48% of the time, Keller did it almost 60%, using it time after time even when it was being destroyed. That’s the difference between a rookie trying to find his balance and a well-established veteran.

That’s why I find really encouraging to read that reportedly, Keller spent the offseason and the delayed start doing two important things: first, working with Rapsodo’s data to evaluate his shortcomings and, second, developing a changeup which will surely help to add to his pitching arsenal as it will complement the fastball, which he pretty much needs to. And as a big plus, his (k-bb)/ip last year was in the top 15 (N#11) for pitchers with at least 45 IP and 10 games started, very similar to those of Shane Bieber and Blake Snell.

The key factors for Keller this year will be: use less of a better-located fastball, more of the slider and curveball, and pepper with the changeup. That way his luck might turn around and his talent will impose. If Pittsburgh’s defense improves (as it looks like it will) then it will add to the mix of a possible Rookie Of the Year candidacy for Keller.

Keller is still available in more than 50% of Yahoo! Fantasy Baseball leagues.

All data used was taken from,, and/or, unless otherwise stated different.

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