The Red Sox built up their bullpen to an impressive degree this offseason, and it was fully expected to be among their biggest strengths in 2016. So far, they’ve mostly lived up to that billing with just a few blips on the radar. Heading into Sunday’s action, they were eighth among all MLB bullpens in ERA and fourth in FIP. They’ve been a big help for a rotation that has certainly had its moments, but has also had its inconsistencies. The talent in the back of the bullpen is particularly jaw-dropping, and has performed exceedingly well this year. Koji Uehara has been a big part of this success, but he also hasn’t really looked like the same old Koji.

To be clear, this is not a piece that is going to say that Uehara is bad now. He very clearly isn’t, and I’m not convinced he has the ability to be bad. He’s been more than acceptable to start the season, pitching to a 2.93 ERA and a 2.16 FIP. Of course, we’re dealing with a 15-1/3 inning sample, so that last number is a bit misleading. Specifically, it is heavily influenced by the zero home runs allowed by Uehara so far this year. Still, he’s been good, but watching him feels different. He doesn’t quite look like the same dominant Koji we’ve grown to love over the last few seasons.

Very quickly, let’s touch on that lack of home runs. He’s still employing a fly-ball heavy approach, with his 34 percent ground ball rate coming in exactly one percentage point above his career average. That’s a whole lot of balls being hit in the air, and eventually a few will leave the yard. At that point, the FIP will come up, and something will have to change if he wants to stay in the same elite range.

The glaring change that needs to be made is with his control. Throwing strikes has been Uehara’s calling card throughout his career, as he’s never allowed more than two walks per nine innings in his career. That is, until this season when he is allowed 3.5 free passes per nine. Again, we’re dealing with a small sample, but this isn’t a one-outing issue. He’s allowed a walk in five of sixteen outings this year after only having eight such outings in each of his other campaigns with the Red Sox. Obviously, that’s not a very encouraging pace.

It’s not just the raw walk numbers, either. They are just a side effect of the biggest issues. The first is that he’s simply missing the zone more often. His 47 percent zone rate (per Baseball Prospectus) is just the second time he’s carried a rate below 50 percent. For what it’s worth, the only other season was 2015, so this isn’t exactly a new trend. Missing the zone isn’t the main concern, though. What’s really changed is what’s happening with those pitches that miss the zone.

As anyone who has watched Uehara consistently over the years knows, his bread and butter is getting hitters to chase pitches that fall out of the strike zone. It was almost a religious experience watching him do that to batter after batter with his splitter in 2013. This year, batters are chasing balls out of the zone from Koji much less than in the past. The 36 percent O_Swing_RT he is allowing would be the second-lowest of his career and easily his lowest since joining the Red Sox. During that time he’s hung around a 40 percent rate. The main reason for this issue can be found with that vaunted splitter.

In a cursory look at how the pitch has changed in 2016, the drop in velocity immediately jumps off the page. It’s down a couple miles per hour from his 2013-2015 average speed, and is coming in a little under 80 mph according to Brooks Baseball. Obviously, that isn’t helping anyone but the hitter. At the same time, however, Uehara has never been a guy to rely on his velocity, so there must be more going on here.

Since I’m still writing, it’s pretty obvious that there is. To put it simply, the pitch just isn’t dropping out of the zone anymore.

As you can see, Koji is getting less movement on the splitter than he ever has in a Red Sox uniform. Intuitively, we all know that a flat splitter makes everything a whole lot easier on a pitcher. Whereas before they were constantly guessing whether or not it would stay in the zone, the flat pitch makes it a lot less challenging to decide when to swing the bat. It really shows up in the swing rates by area, too.

That gif is pretty self-explanatory, with Uehara seeing significantly fewer swings on pitches below the zone this year, particularly on pitches away from right-handed hitters. From what I can tell, this is the biggest reason for his increased walk-rate. It’s not so much missing the zone, but hitters having an easier time to lay off the pitches that miss the zone.

As I said above, Uehara isn’t bad all of a sudden. He’s still getting plenty of strikeouts — 10 per nine innings — and is still inducing plenty of weak contact. There has been some visible decline, however, as can be expected for a pitcher in his age-41 season. Really, this is all the more reason to be excited about Craig Kimbrel’s presence. Uehara could and should be a key member of the bullpen for the remainder of the year, but age is catching up to him and his masterful splitter. It’s reasonable to look for a few more shaky outings from him than one typically expects. Thankfully, he has more help in the bullpen than ever before.