That dude had absolutely no business being on a big league field. Some good stuff but not at all refined.

Yup.  When you have stuff like he does, pitching isn't rocket science.  But it's also not grenade-fishing.   Guys like him are why the upper minors exist.

Like, last year there was a dude on the team in my bunghole AA city.  24, Big-league sibling, big-league stuff, A-ball location at the beginning of the season.  Results were fine when he managed to keep the fastball stay out of the middle of the plate and the slider out of the dirt and at least looking like something to swing at, which didn't happen often together.  He sometimes survived the rest of the time on pure stuff and guys fouling off or popping up mistakes. 

But by about July, suddenly the fastball had settled down and the slider was going from outside at the knees to way outside at the ankles and getting swing after empty swing.

He has a 2.05 ERA in the majors this season. 

But sending him out in the majors last year would have been a disaster.  Every mistake would have ended up in the gaps or in another state and he wouldn't have gotten the learning opportunity.