Jake Arrieta’s ERA is not what it seems to be

Jake Arrieta has been a frustrating pitcher to evaluate since his 2015 Cy Young season. He had a sub-2.00 ERA — though his RA9 was over 2.00 — a 27.1 K% and 5.5 BB% that year. That added up to 8.3 WAR, and his performance won him a very competitive National League Cy Young award. His two seasons since then were not nearly as dominant.

I am sure that Arrieta wished he could have hit free agency after his stellar 2015 season. It was not a complete outlier season, either. He was quite good the year before, with similar peripherals and a 2.64 RA9, which was good for 5.4 WAR. Had he been able to hit the market after 2015, he might have gotten twice the years and dollars of his current three-year, $75 million deal.

It is hard to say for sure given how odd free agency was this past offseason, but had Arrieta continued to be close to the pitcher he was in 2015 over the subsequent two seasons, he probably would have easily beaten Yu Darvish for biggest contract awarded to a pitcher. As you are probably well aware, Arrieta was not even close to the ace he was in 2015.

A couple of things working in Arrieta’s favor in 2015 were a high strand rate and one of the better defenses in the league behind him. He also had a .246 BABIP, but it was possible that there was at least some skill behind his management of weak contact. He had a slider/cutter hybrid that opposing hitters just could not square up. All that being said, his 2.09 DRA was almost identical to his RA9 that year.

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Arrieta predictably regressed in 2016. A 3.28 RA9 is still very good, but he benefited from a historically good Cubs defense that year. That is likely why his BABIP was lower at .241. His strikeout rate decreased by a few percentage points, and more alarmingly, his walk rate shot up to almost 10 percent.

As described by Eno Sarris at the end of the 2016 season, Arrieta’s slider/cutter hybrid had lost a lot of its effectiveness. That was a key factor to his success the year before.

In 2017, Arrieta’s contract year, he regressed even further. He improved his control a bit, but his run average rose to a 4.38 RA9; he was barely an average pitcher. Perhaps most concerning was the fact that his average fastball velocity dropped by approximately 1.5 mph. That electric slider/cutter of his lost even more effectiveness. Opposing hitters slugged .526 against it with seven home runs, per Brooks Baseball. Of course, a pitch’s effectiveness is affected by the pitches that came before it in a plate appearance, but those results are a far cry from the .276 slugging percentage he allowed against that same pitch in 2015.

It should be noted that Arrieta struggled with hamstring problems last year, so that likely affected his performance at least a little bit. That being said, if I were a major league general manager, I would not have wanted any part of Arrieta at the price tag he likely would have commanded in the open market. As Jeff Sullivan noted earlier this year, the fact that the Cubs seemed completely uninterested was also concerning. Put all that together and Arrieta’s contract seems quite reasonable for a pitcher who was about to turn 32 years-old.

On the surface, Arrieta’s 2.66 ERA might look like he is having his best season since he won his Cy Young award. Dig just a little bit deeper, and it is easy to find that his ERA is not telling the whole story. ERA never does, but it is hiding more than usual here.

If you have any familiarity with my baseball writing over the years, you have probably noticed that I never use ERA. I always use RA9. It is simply ERA with the unearned runs factored in. A whole other article could be written on why everyone should use RA9 instead of ERA, so I’ll keep it short here. It is likely nothing you have not heard before. Errors are very subjective, and they have been an outdated form of measuring defense since the early days of baseball. Seriously, it’s been that long. There is a reason why Baseball Reference uses RA9 and not ERA in its WAR calculation.

It is not uncommon to find relievers with a substantial gap between their ERAs and RA9s since they pitch fewer innings. It is pretty rare among starters, though. Last year, Sonny Gray had a 3.43 ERA when he was traded to the Yankees, but his run average was at a 4.45 RA9.

Arrieta’s 3.50 RA9 is almost a run higher than his ERA. That is because the Phillies’ defense has been terrible. The better way to look at that, though, is that Arrieta still has an RA9 that is quite good despite that terrible defense. When you have a 56 percent groundball rate, a bad defense can be extra damaging. Still, having 1.8 WAR one third into the season is quite impressive. It is in the top ten in the NL.

Despite having a good RA9 so far this season, his peripherals are pointing in the wrong direction. He has a 5.9 percent HR/FB ratio which is nearly half of his career rate. More concerning is that his strikeout rate has fallen down to 17 percent. That ranks in the bottom 15 in baseball among qualified starters. Michael Beller goes into greater detail on Arrieta’s strikeout problems over at Sports Illustrated.

That low strikeout rate is contributing to Arrieta’s mediocre 4.15 DRA. He had a 4.17 DRA last year. To be clear, DRA is an RA9 estimator that does factor in the Phillies’ poor defense. He is being evaluated as no better than an average pitcher.

If Arrieta is just an average pitcher now, that might sound like a problem, but it really is not, even when factoring in his salary. If you are a Phillies fan who is bummed out about that, remember that an average starting pitcher is still a valuable thing to have, especially for a team that might be ahead of schedule on its rebuild. And there is still that upside to dream upon.