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One Big Question: Can Jake Diekman keep the ball in the yard?

The lefty has incredible stuff, but last year was a worrisome outlier in one way.

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MLB: Spring Training-Boston Red Sox at Minnesota Twins Sam Navarro-USA TODAY Sports

Welcome to the annual Over The Monster One Big Question season preview series. Over the next 40(ish) days, we will be running through every player on the Boston Red Sox 40-man roster and identifying a key question for them pertaining to the coming season. We will go through the roster in alphabetical order. For the most part, these will run Monday through Friday every week running up to the week before Opening Day, at least as things are scheduled right now. Obviously, the lockout may change the timing of the season, and it also means we will likely see more additions of new faces. If need be, we will add some weekend posts to fit any and all additions to the 40-man before Opening Day. You can catch up with every post by following this link. With that, today we cover Jake Diekman.

The Question: Can Jake Diekman get the ball back on the ground?

Yesterday we covered our first new (i.e. post-lockout) acquisition in our One Big Question season preview series, looking at Matt Strahm. The southpaw was a major-league signing and thus is expected to contribute in some capacity, but he’s more of an upside play than a big addition. If there was a big addition for the back-end of this bullpen, it was in fellow lefty Jake Diekman, who signed a two-year deal with Boston shortly after Strahm had completed his contract with the club.

In Diekman, the Red Sox get a veteran southpaw who has shown consistently over the years that he can miss bats, induce weak contact, and generally keep runs off the board. The lefty, who enters his age-35 season in 2022, has bounced around the league over his career, starting things out with the Phillies before stints with the Rangers, Diamondbacks, Royals, and most recently with the Athletics. He’d been in Oakland since midway through 2019, and the last two seasons has seemingly found a new level. Diekman was straight-up dominant in the COVID-shortened 2020 season, finishing with an ERA under 1.00 and a FIP under 3.00, while last season he was more good than otherworldly, with an ERA of 3.86.

The FIP was a bit off that mark in the wrong direction, though, largely for reasons we’ll get to in a minute, but I first quickly want to highlight why the Red Sox added Diekman and what he’ll bring to this bullpen. It really just comes down to pure stuff. Equipped with a fastball that sits in the mid-90s and can get up near triple digits when needed, along with a sweeping slider and the occasional sinker, he misses bats at a high rate. He has finished each of the last three seasons with a strikeout rate of at least 30 percent, and never in his 10-year career has he struck out fewer than 25 percent of his opponents. For context, the average strikeout rate last year was 23 percent, and the average rate in 2012 when Diekman made his debut was 20 percent.

And yet, despite the elite strikeout rates the Red Sox were still able to get the veteran southpaw on an extremely affordable two-year deal worth a total of $8 million. A lot of that comes down to the walk rate, which has also been quite high, which obviously is not a good thing. The 2013 season is the last time he had a walk rate under 10 percent, and even that is not really under that mark if you round since he finished at 9.8 percent. Going back to 2017, his lowest walk rate in that span is 12.8 percent. Again, to talk about league-averages, the average walk rate in baseball last season was a shade under nine percent. That said, that is just part of the deal with Diekman, and you bet that in short stints a walk or two won’t hurt you most of the time given his elite stuff.

What’s more concerning to me here than the walks is a troubling outlier of a stat last season from Diekman with his home run rate. It’s one thing if you are issuing a lot of free passes if you’re also limiting the damage when the ball is actually put into play, which the veteran has done for most of his career. Prior to 2021, he’d never allowed a full home run per nine innings, and he’d only eclipsed the 0.75 mark twice in his career. (League average in 2021 was 1.26 homers per nine, for context.) But last season, he saw that number spike all the way up to 1.48 per nine innings after giving up 10 home runs over 60 13 innings. In his nine seasons before that he’d allowed 24 homers.

Now, even over a full season a reliever’s sample size can still be considered small, and sometimes a home run spike is simply some extreme noise being experienced over a smallish sample. But with Diekman, it wasn’t just a few more fly balls sneaking over the wall. Instead, it was just way more balls in the air. For most of his career, Diekman was something of a ground ball specialist, finishing with a ground ball rate of at least 50 percent in four of his first six seasons in the majors. Granted, hitters around the league have changed significantly since that time and they’re looking to launch balls more than ever, but even in the shortened 2020 season his ground ball rate was over 60 percent. Last season, it was all the way down at 35 percent, eight percentage points lower than his previous career low.

The strange thing is it’s difficult to find exactly where this is stemming from beyond recognizing that it’s an issue with his four-seam fastball. That pitch allowed eight of the 10 homers from his opponents last season, and its average launch angle went from four degrees in 2020 all the way up to 19 degrees last season, easily the highest of his career. That said, there wasn’t any major change. The spin rate was in line with his career norms, as was the typical pitch placement and the movement on the pitch.

Minnesota Twins Vs. Boston Red Sox at JetBlue Park Fenway South Photo by Barry Chin/The Boston Globe via Getty Images

The one possible adjustment I can see as a potential alleviator of this issue is adding his sinker back more prominently into his arsenal. That was once his primary pitch, with the four-seam not even appearing (per Baseball Savant) until 2017. Even at that point, though, the sinker was a clear third pitch coming in between 15 and 20 percent of the time. However, the last three seasons it has been a distant third, although with a comeback made toward over the second half of the season, indicating he’s already been trying to make this adjustment.

Diekman, upon signing, instantly became one of the most important members of the Red Sox bullpen, and should be in line for some save opportunities assuming Garrett Whitlock is in more of a spot starter/long relief kind of role. Matt Barnes will be in that mix too, but Boston needs Diekman to be at his best, which means keeping the ball in the yard. Whether the adjustment is throwing more sinkers or simply making a tweak with his four-seam, he needs to figure out a way to avoid a repeat of last year’s anomaly with his home run rate.