What's happened to Washington Nationals starter Jordan Zimmermann?

Using the archaic pitcher wins model, Jordan Zimmermann improved to 10-8 on the year in Sunday's 9-5 win over the Milwaukee Brewers. It was Zimmermann's second straight victory, evening his record at 2-2 this month. Of course, we know that pitcher wins aren't exactly the best way to determine how a guy is pitching. At this point in the year, Zimmermann is clearly having his worst season since 2010, when he pitched in just seven games in his return from Tommy John surgery.

While Zimmermann's recent problems haven't plagued him for the entire year, let's take a quick peek at his overall season performance relative to his past:

Looking at those numbers, the immediate instinct is that we shouldn't panic too much about Zimmermann's performance in 2015. While Zimmermann was among the most dominant starters in the league in 2014, his production last season was a bit of an anomaly. His strikeout rate was 0.79 higher than his career rate. His walk rate (while always a strength) was down 0.49 from his career rate last season. Perhaps most importantly, his home run rate was a career best, as he allowed 0.24 fewer home runs per nine innings last season than he has in his career. All of those things added up to a career best ERA and FIP in 2014.

In 2015, Zimmermann's season-long numbers tell us that he's simply regressed back to being the pitcher that most of the problem is that he's regressed back to the pitcher that he was from 2011-2013. OK... Maybe he's been a little worse, but his 2.6 fWAR to this point in the season suggest that he's on target to finish somewhere between 3.0 and 3.5 fWAR (remember, it's a cumulative stat) assuming that he makes each of his final seven scheduled starts. From 2011-2013, Zimmermann finished with a WAR between 3.3 and 3.6 each season, so it's hard to really be disappointed if he finishes in that range.

Of course, looking at the entire season isn't going to tell us how Jordan Zimmermann is pitching right now. What's kind of frightening about Zimmermann's performance right now is that his season-long numbers have been propped up by a strong first half performance. Actually, they're propped up largely because of an absolutely dominant May (1.91 ERA, 3.24 IP) and a strong June (3.00 ERA, 2.91 FIP). He's struggled since the second half began (4.28 ERA, 4.32 FIP), particularly in August (4.31 ERA, 4.88 FIP). There's been one obvious problem....

Zimmermann has never really had much trouble keeping the ball in the yard. This isn't because he's ever been an extreme ground ball pitcher or anything. He's been average at keeping the ball on the ground, but his 1.21 GB/FB rate since coming into the league in 2009 ranks 179th among 314 qualifying pitchers in that time. His HR/FB rate is the number to look at here, as he's allowed 8.8% HR/FB throughout his career. This is a touch better than the league average, as the league average has hovered between 9.4% and 11.3% from 2009-2015. Generally, the league average falls around 10%.

For the year, Zimmermann's HR/FB rate looks fairly steady in relation to the rest of his career. It's at 9.2%, identical to his 2012 finish and 0.3% lower than where it finished in 2013. It's quite a bit higher than the 6.4% HR/FB rate that Zimmermann finished with last season, but as we mentioned above, last year looked like a bit of an anomaly. Here's where the issue comes to light. Let's look at Zimmermann's batted ball splits so far this season....

After allowing just eight home runs in 111.2 innings in the first half, Zimmermann has already allowed nine home runs in 48.1 innings since the All Star Break. Eight of those home runs have come in his five August starts. Actually, all eight of those home runs have occurred in three starts (two of which the Nats won) against the Mets (3 HR in a 5-2 loss), Rockies (3 HR in a 15-6 win), and Brewers (2 HR in a 9-5 win). He's been getting tagged for an alarming 17.3% HR/FB rate in the second half, including 22.2% so far this August.

Oddly enough, Zimmermann's batting average (.240) and OBP (.283) allowed so far in August are the lowest figures he's allowed all season long, but his slugging percentage allowed (.483) is far and away the highest slugging he's allowed in any month this season. In fact, it's 72 points higher than his July slugging allowed (.411) and 111 points higher than it's been in any other month this season. I'd say that these numbers seem unsustainable (the HR/FB rate certainly does, as it's more than twice the league average this month, and his second half numbers overall are nearly twice his career rate), but it's troubling that we see a trend where his slugging percentage allowed has risen significantly (.328 in May, .372 in June, .411 in July, .483 in August) in each of the past three months.

Allowing more home runs per fly ball allowed is a trend we don't want to see in any circumstance. It's particularly worrisome when it's accompanied by a rising fly ball rate (37.3% this season, 1.4% higher than last season, 1.9% above his career average). The immediate assumption is that Zimmermann must be having some command issues and has left the ball up in the zone a bit more than he has in the past. I'm not an expert on pitching mechanics, but I have to wonder if there's a mechanical issue that's affecting Zimmermann's command.

Pitch Types/Values

The last thing we're going to look at is Zimmermann's pitch value in terms of Runs Above Average according to Fanduel. We'll notice the biggest issue in the very first column.....

Zimmermann's fastball has been a key to his success throughout his career with the Nats. Much like the rest of his 2014 campaign, Zimmermann's 14.6 RAA with his fastball last season seems to have been an anomaly (OK... the 12.3 with the slider was as well), but he was consistently above 6 RAA from 2011-2013. This season, his fastball has netted league average results, at just 0.4 RAA. His slider has returned to 2012-2013 performance, which was probably to be expected.

Zimmermann's fastball hasn't really lost much life. His velocity has dropped from 93.8 MPH in 2014 to 93.0 MPH this season. It's certainly a decrease in velocity, but it's not a massive dropoff like we occasionally see as a pitcher ages. Zimmermann has generally maintained his velocity on all of his other pitches as well. His vertical and horizontal movement haven't really changed on his fastball, which again tells us that command is the likely issue.

Over the past couple of months, Jordan Zimmermann hasn't been the same pitcher that we've been watching in D.C. for the past six seasons. It's primarily due to a spike in his home run rate, which can be partially ascribed to command issues and partially ascribed to some bad luck. With just seven regular season starts remaining (most likely) in his tenure with the Nats, there's not much time for the Nats to help correct the issues that have been plaguing him for the past month. We're probably going to have to hope that he finds a groove and his recent run of poor luck regresses positively.