18 year old Cuban pitcher Gerardo Concepcion has moved to Mexico and will become a free agent. So I found Cuban pitchers who pitched both professionally in Cuba and the MLB. I could find just 3 with reliable statistics.
Cuba: 10.04 K/9IP, 3.75 ERA
MLB: 12.8 K/9IP ,3.27 ERA
Cuba: 7.6 K/9IP, 2.45 ERA
MLB: 4.14 K/9IP, 5.52 ERA
Cuba: 14.97 K/9IP, 4 ERA
MLB: 6.8 K/9IP, 4.54 ERA
Contreras and Maya saw significant decline in the Majors, while Chapman saw improvement. The biggest difference between Chapman and Contreras/Maya is that Chapman throws an average 99 MPH fastball, hitting 100 frequently, and neither Contreras or Maya hit 95. As I had shown in a previous article, pitchers that threw 95 MPH were much more likely to have success than pitchers that didn't. According to reports I have seen, Concepcion can barely hit 90 MPH, which is about what Maya throws. So here we will look at Concepcion's statistics through Contreras and Maya's difference in the two countries.
Cuba: 4.7 K/9IP, 3.36 ERA
MLB Projection: (negative strikeouts somehow), 5.165 ERA
Just for fun, a projection including Chapman: 1.74 K/9IP, 4.4 ERA
As you can see, the strikeout projections are particularly unhelpful, but a 4.7 K/9IP at any level is terrible. His ERA is also very pedestrian and unimpressive. He is only 18, and could develop more (I would be surprised if he didn't), but throwing 90ish with no apparent strikeout abilities isn't exactly something teams should be clamoring over.