Speier goes into more detail here:
https://www.bostonglobe.com/sports/redsox/2017/12/18/what-left-red-sox-farm-system-here-are-top-prospects/zPy507YNIMZcgt19C15v4H/story.html
Darwinzon:
WHY HE’S RANKED HERE: Few in the Red Sox system can match the present stuff of Hernandez, who unleashes mid-90s fastballs (topping out at 97) from a low three-quarters arm slot. He’s leaned heavily on a curveball throughout his career, but in his final start of the year for Greenville, he finally committed to a slider that looked like an absolute wipeout offering
PROJECTION: If everything clicks and he realizes how to employ a full three- to four-pitch mix (fastball, slider, curveball, changeup), Hernandez could be a starter in the mold of Francisco Liriano, albeit one who is likely capped as a mid-rotation starter by his limited command. At the least, he has the power stuff and aggressive attack plan – he hunts swings-and-misses with his fastball – to be a formidable back-end reliever.
QUESTIONS: Will Hernandez continue the sort of control gains he made in 2017, when he dropped his walk rate from 6.7 per nine innings to 4.3 per nine? If not, can he ever assume the innings total necessary to be a starter? Will he make the necessary progress to realize how to sequence his pitches to thrive, or is he more of a let-it-fly guy who is suited for the late innings?