Luis Castillo appeared on fantasy sleeper lists so often this preseason that he probably stopped being an actual sleeper.

If the first half was any indication, though, owners would have been better off to keep sleeping. The Reds' righty has been wholly underwhelming in 2018, given his dazzling rookie campaign in 2017.

In 103 1/3 innings this season, he has a 5.49 ERA (134 ERA-) after pitching to a 3.12 ERA (71 ERA-) in his first 89 1/3 big league frames.

It was not realistic to expect Castillo to post such a sterling ERA again, considering his 2017 number had the benefit of an unsustainably low .247 BABIP and an unsustainably high 80.1% strand rate. This season, his BABIP has regressed to around the mean (.303) while his strand rate is now an outlier in the other direction (66.7%).

His poor results in 2018 go beyond mere regression in terms of sequencing and batted-ball luck, however. His peripherals have also taken a hit, as he posted a 3.74 FIP (85 FIP-) as a rookie but owns a mark of 4.75 (114 FIP-) this year.

Castillo's walk rate has improved, but his once stellar strikeout rate is now below the league average and his home run rate has increased by more than 0.5 dingers per nine innings.

What should we expect from Castillo moving forward?

Keep the Whiffs Coming

Castillo's strikeout rate is just 21.5% in 2018, but the modest results betray a solid process.

He is throwing considerably more first-pitch strikes than he was last year and is getting opponents to both chase more pitches out of the zone and make less contact in the zone, according to FanGraphs. Overall, opposing batters are whiffing more on a percentage of their total swings and total pitches faced.

His contact rate has fallen from 73.3% to 71.6%, while his swinging-strike rate has climbed from 12.2% to 14.0%, the sixth-highest rate in the league.

Swinging strike rate is one of the best indicators for strikeout rate, as the two stats are highly correlated (this season, the correlation coefficient is 0.87, with 1.0 implying direct correlation and 0 implying no correlation).

Castillo is a massive outlier, as you can see in the chart. Given his swinging-strike rate, we would expect his strikeout rate to be about 29.6%, more than 8 percentage points higher than what it actually is.

Swinging-strike rate tends to be stable over time and is a good predictor of future strikeouts (last year, first-half swinging-strike rate correlated with second-half swinging-strike rate at 0.76 and second-half strikeout rate at 0.66; strikeout rate correlated with itself from one half to the next at 0.73).

On the dazzling swinging-strike rate alone, Castillo is worth an inclusion in our midseason buy-low list for season-long formats, even if there's still plenty of risk here.

Couple his whiffs with a 7.9% walk rate, and Castillo is in good shape -- if not for a dramatic spike in home runs.

Home Run Troubles

Castillo's innings pitched total has increased by about 16% from last season to this one, while he has given up almost 73% more home runs. Last season, he allowed 11 big flies, and he has surrendered 19 jacks in 2018.

A pitcher's home run total can be flukey in small samples, as home-run-per-fly-ball total is prone to a ton of random variation. Generally, the batter has more control than the pitcher over whether the ball leaves the yard, so the most reliable way for a pitcher to avoid giving up home runs is to prevent giving up fly balls in the first place.

Castillo did well in this regard last year, as his 58.8% ground-ball rate was seventh-highest among the 167 pitchers to throw at least 80 innings.

This high grounder rate helped him post a better-than-average home run rate, despite a high 17.2% home-run-per-fly-ball percentage.

In 2018, Castillo is allowing an even higher rate of fly balls to leave the yard (18.4%) and is conceding more flies in general -- his ground-ball rate has dropped to 44.6%, which is around the MLB average.

It is hard to pinpoint exactly what has prompted this decline -- Castillo's pitches have roughly the same amount of vertical movement this year compared to last year, while the average locations of his fastball and sinker are very similar. He appears to be throwing his slider up in the zone a bit more, but this is only a pitch that he has thrown about 15% of the time.

There does seem to be something real going on here, though, as we can infer that a pitcher's ground-ball rate is more of a product of skill than randomness after about 70 balls in play (Castillo has allowed 307 balls in play this year).

This is problematic because Castillo is getting hit incredibly hard. His hard-hit rate is much too high, sitting at 38.3%, and Castillo is in the sixth percentile in terms of "barrels" per batted ball (a barrel is contact with an expected average of .500 and slugging percentage of 1.500, based on its launch angle and exit velocity).

Also, based on the Statcast data for the balls in play he has allowed, Castillo would have been expected to allow 18.6 home runs in the first half, per xStats.org. Andrew Perpetua, the proprietor of xStats.org, found that expected home runs are predictive of future (actual) home runs.

We should still expect some regression in terms of home-run-per-fly-ball rate, since it is such a volatile stat for pitchers and hitters generally have more say over batted-ball authority than pitchers.

However, given how hard Castillo has been hit, it is hard to see this rate regressing all the way down to the league mean, making it imperative for him to get the ball back on the ground to keep the ball in the yard.

Castillo could see his strikeout rate jump in the second half, but if he cannot get his home run issue under control, it won't matter much.