Jake Arrieta’s Elevated Walk Rate

When looking at Jake Arrieta’s season thus far, it’s really hard to complain. His 1.74 ERA and 0.98 WHIP are virtually indistinguishable from his 2015 combo of 1.77/0.86. Expanding out further, the 28% K rate, 0.3 HR/9, .244 BABIP, and 57% GB rate look virtually identical to the 27%, 0.4, .246, and 56% marks from last year. There has been one stark difference, though. Arrieta’s walk rate has climbed from 6% to 9% this year. To put a finer point on it, he walked 48 batters in all of 2015; he has 40 already this year.

So what’s going on and should we worry at all?

First place I usually look with walk issue is first-pitch strike rate. Is Arrieta falling behind early and losing more hitters? The answer to that is a flat “no”. His 60% rate is the same as it was last year and if you move the decimal out, he’s actually a tiny tick better at 60.4% compared to 60.2%. From there I go to the chase rate. Are batters spitting on more pitches outside of the zone? That is definitely the case so far this year. His O-Swing rate was 34% last year, good for 11th in baseball. He’s down to 29% this year, just 49th among the 96 qualified starters.

Of course, that doesn’t give us an answer. We can’t just say “they’re swinging less” because… no shit. What is underneath that lowered chase rate? On MLB Network, they highlighted the cutter/slider/slutter (call the pitch whatever you want – it likely morphs between all three classifications) to righties as part of the problem, suggesting that it’s such a difficult a pitch to hit that instead of trying, they’re just laying off.

The idea isn’t wrong, but it’s not the driver behind the surge in walks. They still can’t hit it, but they are laying off more. Righties had just a .572 OPS with a 5% BB rate against it last year; the OPS is down to .539 this year, but the BB rate is up at 15%. However, with just 54 PA, we’re talking about eight walks, which just so happens to be how many he allowed over the 175 PA in 2015. The bigger issue is the fastball against lefties.

Last year lefties hit .199/.266/.284 off Arrieta’s heater in 154 PA with an 8% BB rate. This year, they’re at .241/.394/.333 in 110 PA with a 20% BB rate. They chased the pitch at a 28% clip last year, but it’s down to 23% this year. So despite putting the pitch in the zone more often (54% to 49%), the walks are much more plentiful. In raw numbers, they’ve nearly doubled from 13 off the heater last year to 22 already this year. It’s the tough fastballs against lefties that are going the other way, too.

There is a measure of “competitive pitches” which are those within 18 inches of the center of the zone. Last year, they chased 46% of Arrieta’s competitive fastballs. It’s down to 29% this year. Looking at his strike percentages outside of the zone shows that lefties have improved on pitches both high and low. Middle-out is the only spot where he’s really gotten better against lefties:

At its core, this feels a lot like what has happened to Dallas Keuchel early on: batters take the unhittable stuff much more to force him back in the zone. The sharp difference between these two is that Arrieta’s stuff is good enough to succeed in the zone. Last year, Keuchel had a .736 OPS on PA ending in the strike zone, .339 out of zone. This year, he’s at .900/.594. Arrieta has gone from a .524/.461 in/out combo last year to .509/.505 this year.

Perhaps the elevated walk rate against lefties would be more problematic if he wasn’t smothering righties so much. He has essentially flipped his platoon splits this year, going from .557 vs. right/.449 vs. left last year to .454/.592 this year. These walks aren’t a change in skill level, but rather an adjustment that the league – specifically lefties – has made against him on certain pitches. It’s not like they’ve figured him out. Even isolating June, which has been his worst month against lefties, leaves him better than average with a .673 OPS compared to the .756 average (min. 50 PA v. lefties). His 19% BB rate is dead-ass-last among the 71 qualified arms, but his 32% K rate is fifth best.

The walks aren’t symptomatic of skills degradation, rather mostly just a shift in baserunner distribution from hits to walks. Consider that Arrieta is one of five pitchers with at least six 4+ BB games this year. His 2.25 ERA and 1.22 WHIP combo in those games is easily the best of the bunch:

• Francisco Liriano – 9 games, 7.00 ERA, 2.04 WHIP
• Tom Koehler – 7, 2.57 (lolwut?), 1.64
• James Shields – 6, 7.16, 1.87
• Ubaldo Jimenez – 6, 7.20, 2.00

To sum up our original question, what’s going on is that lefties are chasing a lot less, but they haven’t converted that patience into significantly more damage, so no, we shouldn’t really worry about this.