Last September, a starting pitcher emerged who would go on to be one of the more popular under-the-radar “breakout” picks of the winter. Acquired from Miami in the Mat Latos trade, Anthony DeSclafani was a 24-year-old with middling results across his 31 starts last year, including a 4.05 ERA and a 12.2-point strikeout- and walk-rate differential (K-BB%). The results on their own were more than serviceable for an innings-eater type pitcher, but they weren’t exactly exciting. Until you looked at September, that is.

In September, his ERA clocked in at an unappealing 4.93, but the underlying peripherals were fantastic. He struck out 24.8% of batters while walking just 3.4% and maintained a solid 47.1% ground-ball rate — all of which left him with a 2.27 FIP for the month. A bit of bad luck in batted balls and sequencing made it possible for numbers-friendly fans to uncover what really happened with DeSclafani that month and feel like you were unearthing a great secret, because not only was DeSclafani putting on a hidden great performance, it was accompanied by the always enticing logical explanation.

Take a look at DeSclafani’s pitch-usage chart from 2015 and you’ll find that, at the end of the season, he largely scrapped his changeup and dramatically increased his curveball usage. Additionally, he decreased his reliance on the four-seamer. Great run-prevention numbers may not have initially accompanied the adjustment, but the peripherals indicated that DeSclafani had taken a step forward and could be in store for a strong 2016 campaign. And, as it turns out, that’s exactly what’s happened.

An oblique strain at the start of the season kept DeSclafani from making his 2016 debut until June 10th, but since his return to a major-league mound, he’s been fantastic. He’s made 17 starts and he’s yielded three earned runs or fewer in all but two of them. Two weeks ago, he pitched the first complete-game shutout of his career. And, what’s more, he’s maintained the adjustments that he made at the end of last season.

DeSclafani found a pitch mix that has led him to success, but do the peripherals tell the same story that caused people to buy-in last winter or has the pendulum swung so far that now the results are outperforming underlying indicators?

A quick glance at basic indicators will indicate that there could be a bit of overperformance at play here. DeSclafani has a stellar 2.93 ERA this season, which ranks 13th out of the 127 starters who’ve thrown 100-plus innings this season. That’s great! However, his FIP (3.81), xFIP (3.91), and SIERA (3.91) are eerily consistent in showing that the peripherals don’t provide full confidence that his current success is sustainable. Let’s start with a look at the positive takeaways, though.

One of the great improvements with DeSclafani’s adjusted arsenal was his dual ability to strike batters out and avoid walking batters. It was a big part of his step forward last September and it’s something he’s been able to maintain through the 2016 season.

Although his overall rates — 21.7% K, 5.1% BB — aren’t quite league leading, they’re still good enough for his K-BB% rate to fall in the 75th percentile among major-league starters this season (min. 100 IP). His curveball has been a real strikeout weapon for him thanks to its whiff-inducing abilities. According to Baseball Prospectus’ PITCHf/x leaderboard, the 41.8% whiff/swing rate on DeSclafani’s curve ranks 12th of 79 starters with 200-plus curves thrown this season. His go-to two-strike pitch, however, is his slider. The pitch has a slightly lower whiff rate and ground-ball rate than his curveball, but by our linear weights it has been his most effective pitch this season. No matter how you look at it, he has two strong offspeed offerings and it’s helped him maintain solid strikeout and walk numbers.

Unfortunately, not all of the improvements stuck. His ground-ball rate has been steadily falling after peaking last fall.

The biggest culprit appears to be a massive drop in ground-ball rate against his sinker. As a result, it probably won’t surprise you to learn that he’s not commanding the pitch to the bottom of the zone as well as he did a year ago. The two pitch charts below show DeSclafani’s sinker. September 2015 is on the left; 2016, on the right.

But a moderate drop in ground-ball rate isn’t the biggest concern for DeSclafani. No, the biggest concern lies in the fact that he’s a right-handed pitcher who essentially swapped his changeup for a curveball. As you know, the lack of an effective changeup will leave a pitcher susceptible to opposite-handed batters. The hope, of course, for a pitcher like DeSclafani is that his big curveball will help combat that vulnerability. Unfortunately, that hasn’t been the case.

The good news is that DeSclafani has been among the absolute best pitchers in baseball against right-handed batters.

It should be noted that DeSclafani sports an unsustainably low .202 BABIP against right-handers, but that doesn’t make it any less true that he’s been among the most effective pitchers against righties this season.

The bad news is that DeSclafani has been among the absolute worst pitchers in baseball against left-handed batters.

DeSclafani’s breakout was expected to some degree, and there are great things to take away from his performance. He has two legitimate breaking pitches that baffle same-side opponents. However, it’s going to be hard to buy in on him as anything more than an inconsistent mid-rotation guy unless he figures out how to effectively neutralize lefties.