Kenley Jansen isn’t out of the woods yet

From the moment he joined the Dodgers in 2010, Kenley Jansen has been the West Coast version of Mariano Rivera. Like Rivera, Jansen primarily throws a cut fastball. And like Rivera, when Jansen enters the game, you can usually assume it’s over.

From 2010–2017, Jansen ranks second among qualified relievers with 17.3 WAR. The difference between him and the man above him, Craig Kimbrel, is only 0.2 WAR — so small as to be meaningless. If you go by WPA/LI, Jansen ranks first in the same span. Like Rivera, peak Jansen put his opponents to sleep.

But peak Jansen didn’t start the 2018 season. Instead of the Jansen who’d allowed a measly .226 wOBA to that date, we got one who allowed a whopping .354 wOBA. Whereas previously he’d made hitters look like 2017 Zack Greinke, this spring he made them look like 2017 Jonathan Schoop.

Thankfully for Dodgers fans, since May 1st he’s turned the opposition into pumpkins, assuming those pumpkins can muster a .175 wOBA at the plate. This performance, along with the team’s climb back into contention, has taken the heat off. Instead of looking at Jansen, writers are focusing on breakout guys like Max Muncy and Ross Stripling. Heck, even Matt Kemp has drawn coverage for, you know, playing decent baseball.

You might think the danger has passed. But even if you discount his April woes, some signs indicate Jansen isn’t fooling hitters the way he used to. The first stats I always look at for pitchers are their strikeout and walk rates. These indicate how well they’re controlling the strike zone and fooling hitters.

Through the 2017 season, Jansen’s career K-BB rate was 33.3%, tops among all qualified relievers. So far in 2018, though? That rate is a shockingly below-average 18.2%. The following graph shows the stats that comprise this rate in 10-game intervals over the past five-plus years:

Jansen’s K-rate hit new depths this season. After sneaking above the 37% mark for a few games in May, it’s trended downward recently. At the same time his walk rate, once superlative, has struggled to stay where we’re used to it being.

What gives? Jansen’s throwing fewer strikes, and hitters are chasing less often than they have in a long time:

He boosted his in-zone total after his horrendous April, but lately he’s been struggling again.

When batters do swing, they’re making contact at rates not seen in over two years:

As you might expect, his velocity is down. The last time Jansen throw this slowly, we were still wondering whether Mike Trout was for real:

We don’t have a lot of xWOBA data, but this year Jansen’s looked more hittable than in years past:

Besides a drop in velocity, the Dodgers are shifting behind Jansen more than they ever have. Why is this relevant? Russell Carleton found that while shifts take away singles, they are associated with more walks and extra base hits.

He theorizes that some pitchers just don’t feel comfortable with the tweaked defensive positioning behind them. They experience a “psychological stumbling block” that results in fewer pitches thrown in the strike zone. Since he’s a clinical psychologist by trade, I take stock in his opinion.

I couldn’t find any evidence the shift freaks Jansen out. But just look at the following data:

While we can’t prove causation, the correlation is as plain as day.

Lastly: don’t trust that low ERA Jansen’s sporting. It’s propped up by a tiny .209 BABIP. Will that continue? Not likely. I dug into the data; since 1997, 931 relievers have thrown between 65 and 75 innings, which is where Jansen likely will finish. Of these only 21 (2.2%) finished with a BABIP that low or lower.

Dodgers fans may look at Jansen and conclude his early season struggles are behind him. I look at Jansen and see a pitcher who’s scuffling to regain some of his lost elite abilities. He can still be good; after all, a .273 xWOBA isn’t terrible, and his xFIP- of 92 indicates he can still get hitters out better than the average pitcher. A .209 BABIP still means that runs aren’t scoring against him.

But the Dodgers and their fans won’t settle for “better than average”. They want “dominant”, “filthy”, and “lights-out”. Great playoff teams feature lockdown closers and Los Angeles has its sights set on a world championship. After years of playoff struggles and a disappointing, grueling World Series loss last year, the team surely hopes Jansen can recover his elite swing-and-miss stuff in time for October.