Fastball - He has four different kinds.

• Four seamer that he can throw anywhere from 93 to 95. He can throw even harder (up to 97) but rarely does.
• Two seamer with solid downward action. Very consistent action but not the hardest late break. Typically 89 to 92. Sometimes I've seen him as low as 86 and sometimes up to 94.
• Two seamer with decent downward action and good tailing action. Works this pitch well in on the hands of RH batters. Needs to use it inside on LH hitters more often. Could also back-door it on RH hitters more.
• Two seamer with nasty late break. Maybe 2-3 a game where it's just filthy. It's just a variation of the normal 2 seamer, but if he can do this more often watch out.
  

Curveball

• Typically a hammer curve but usually breaks earlier than wanted
• Frequently it just goes high in the zone and doesn't break much. Doesn't drop back into the top of the zone to be a BP pitch. Just a ball
• Can look far too much like his changeup. Only uses it 4-5 times a game
• Will get 2-3 excellent ones a game. Shows enough to me that he should feature it more, get rid of the slider.

Slider

• Rarely uses it
• Backs up on him too often
• Ditch it - I'd rather him throw the curve more and hopefully gain more control

Change

• Very good pitch
• Again has solid or very good downward action
• Occasionally has heavy late break with tailing action. Dominant at times.
• Locates it well.
• Natural progression leads me to believe this could become a dominant weapon for him.

Prognosis: I really like the moving fastball and change-up combo. What I think hurts him is his slider, change, curve, fastball all have similar action. He needs to mix it up. I would get rid of the slider, mix in the curve more. There would be natural progression on the change. And most important - make the four seam fastball a weapon. At the end of the season he started to mix more in.

if you look at fangraphs you can see he has a low swing percent in zone. Everything he throws starts low and drops below the zone. Once a pitcher starts dropping pitches in for called strikes then hitters will start jumping at pitches instead of waiting for grooved pitches.

So start using the 4-seam on outside corners. Start stealing strikes. Back-door the tailing action 2-seamer. And work in that curve more often. The change will get better on it's own. Already a good pitch.

EDIT: One thing very difficult when scouting Porcello is separating the pitches. The actions is similar. His slider/curve/change have similar velocities. And similar motions. Pitch f/x can not be accurate with him unless they are looking in his glove and seeing the grip. I've seen him grip a 2 seam and drop the velocity and it looked like a hanging breaking ball before it got crushed. If I didn't see the grip no way I could have known the pitch. And his change and curve are so close. After watching enough you can see his curve hump up before dropping. Or not. It's hard to detect because he doesn't throw many. And half of what he throws are almost identical to his good change.

He is an excellent reason to listen to as many scouting reports as possible on young pitchers