Luis Castillo's early season success was one of the major bright spots for the Reds so far in 2019. But looking at his overall numbers, outside of ERA, it seems he actually has been pitching around the same as he has been his whole career.
2019
3.70 FIP
3.75 xFIP
2.22 K/BB
0.79 HR/9
Career
4.01 FIP
3.63 xFIP
2.86 K/BB
1.21 HR/9
Basically his K/BB rate is actually worse than his career, thanks to too many BB's, while his HR/9 rate is better, which is due to getting more GB and fewer FB.
His much lower ERA seems to be due to his very low BABIP, .243, but he always has had a low BABIP, with a career .264.
Of course, he had a terrible game yesterday, so maybe that has skewed the numbers a bit, but all games count. But it looks like Castillo is due for a correction at some point this season, and may not be the true Ace we were hoping he would be at the start of this season.